Decayed Master

The thirteenth incarnation of the Master was the last in his original regeneration cycle. Following an incident that resulted in his body being horribly disfigured, he began searching for a way to restore his Time Lord body, with only his intense hatred and burning anger keeping him alive.

While he would find numerous means of extending his life beyond natural limits, he would inevitably revert to his decayed thirteenth form before suffering a conclusive end at the hands of the Ravenous. (AUDIO: Planet of Dust)

Origins
Having regenerated into his thirteenth incarnation at an accelerated rate due to living a life "more rackety than the Doctor's", (PROSE: A Brief History of Time Lords) that had constant pressure and danger, in addition to using some incarnations as disguises, (PROSE: Doctor Who and the Deadly Assassin) the Master eventually found himself trapped as a wraith-like living cadaver, "horribly emaciated", for whom "no regeneration was possible". (TV: The Deadly Assassin)

There were, however, multiple accounts of how this came about.

According to one account, the Decayed Master emerged as a result of Susan Foreman forcing the "UNIT era" Master out onto Tersurus and attacking him with the Tissue Compression Eliminator while he was holding the Daleks' matter transmuter. (PROSE: Legacy of the Daleks)

According to Missy and the Thirteenth Doctor, the Master ended up in his rotting, lich-like state when he attempted to regenerate "one time too many" past the twelve-regeneration limit. (PROSE: Meet Missy!, The Doctor vs the Master)

According to yet another set of accounts, the "UNIT era" Master, not yet out of regenerations, began to regenerate after being injured in a temporal storm after an encounter with the Twelfth Doctor. (COMIC: Doorway to Hell) Because he was suffering from an artron energy deficiency (PROSE: The Dead Travel Fast) as a result of the storm, his regeneration went wrong (COMIC: Doorway to Hell) and his new incarnation was brought into existence already horribly scarred. (PROSE: The Dead Travel Fast)

Yet another account showed that the Thirteenth Master, as distinct from the "UNIT era" version, had enjoyed some span of existence in un-disfigured form before being injured in events involving his own future self. (AUDIO: The Two Masters)

At war with his future
According to one account, when the Master arrived on a Time Lord base on Tersurus with the intention of infiltrating their database, he had not been disfigured yet. He encountered a future version of himself, who burned his body, leaving him a fraction above death. The future Master then signalled Gallifrey for Goth to arrive, but before he could arrive, the Cult of the Heretic switched the Master's mind with that of his future self. (AUDIO: The Two Masters) While he remembered being in great pain, the process left gaps in the Master's memory. He returned to a prison in the south of England, where a former incarnation had lured a Carmentine Mind Leach, and set up the Dominus Institute in order to lure the Doctor to him, planning to absorb his intellect. The Sixth Doctor made a deal with the mind leech, which agreed to take only the Doctor's short term memory. This was enough to sustain the Master (AUDIO: Vampire of the Mind) and allowed him to regain his memory of what had happened to him. The Decayed Master then hired the Transhuman Sisters of the Unholy Protocol and the Dragonhunters to kill his future self. (AUDIO: And You Will Obey Me)

The act of having a Time Lord inhabiting the body of his past self led to the universe beginning to break down. Realising this, the Seventh Doctor encountered both of the Masters and persuaded them to return to their rightful bodies. Abandoning the Doctor on a ship that was set to crash into a hypergate, the two Masters stole his TARDIS and returned to the Cult's headquarters, where they killed all of the members and plotted to use the anomaly cage to remake reality in their image. They were stopped by the Doctor, who had escaped the crash in the Master's TARDIS: he used the cage to restore the universe, leaving it practically unchanged, apart from erasing the Masters' memories of the events and sending them back to their proper points in history. (AUDIO: The Two Masters)

Revenge on Gallifrey
Goth collected the Decayed Master, who promised him power, and took him to Gallifrey. (PROSE: Legacy of the Daleks)

Whilst on Gallifrey, the Master made Goth, in line for the position of Lord President of the High Council of Time Lords, into his slave, continuing to promise him power. The Master also took over the Matrix, and realised that the Eye of Harmony resided beneath the Panopticon. He believed he could use the Sash of Rassilon to protect himself from the raw power of the Eye and channel that energy to renew himself.

With a telepathic summons and a vision of the future created by the Matrix, the Master lured the Fourth Doctor to Gallifrey, seemingly to prevent the murder of the then-serving Lord President. The Doctor failed and ended up on trial for the President's murder. After Goth died, the Doctor defeated the Master in physical combat and, as a result, the Master fell into a crevice created by a localised earthquake. He gained access to his TARDIS in the confusion and escaped, (TV: The Deadly Assassin) having been able to convert the energy from the Eye of Harmony and partially heal himself. (AUDIO: Trail of the White Worm)

Targeting Jago and Litefoot
The Master followed the Doctor's TARDIS from Gallifrey, but was knocked off course passing through the transduction barrier. (AUDIO: Masterpiece) Losing control of his TARDIS, he crashed on Earth, 1890, on the shore of Whitby. Bram Stoker saved him from drowning and retrieved his TARDIS. The Master then hypnotized Stoker and forced him first to carry the TARDIS in the ruins of the nearby Whitby Abbey, and then to hunt down for him little animals, whose life energy he sucked dry to sustain himself. Stoker however succeeded in resisting the Master's influence when he ordered him to bring to him a human life, and walked away. He then returned to stop the Master from taking the life of a young girl, and fought with him inside his TARDIS. During the fight, Stoker accidentally activated the dematerialization circuits, sending the ship away. (PROSE: The Dead Travel Fast)

The Master landed once again on Earth, London, in the 1890s. He asked the locals if he could speak to someone in authority, and Inspector Quick was the first on the scene. The Master wasted no time in hypnotising Quick for his goals, (AUDIO: The Museum of Curiosities) giving him several tasks to carry out on his behalf. (AUDIO: Jago & Son) The Master walked the streets of London, concealing himself with a mask and walking with a cane. He spoke to Maurice Ravel, interested in a watch he carried with the Prydonian Seal on it. The Master later learned that Jago and Litefoot were the Doctor's contacts in the time period, and intended to visit them. (AUDIO: Maurice) The Master had Quick bring him a sample of Jago and Litefoot's DNA, planning to poison them so that they would summon the Doctor to help them. (AUDIO: The Woman in White)

As he became more frail, the Master planned to use the Doctor's artron energy in order to heal his form. He travelled to the Red Tavern and spoke to Ellie Higson, hypnotising her so that he could learn of Jago and Litefoot, and reverted her metabolism to its natural state, causing her to lose control of her vampiric hunger. Jago and Litefoot learned of the location of the Master's lair with the help of Madame Sosostris and encountered the Master there. Sosostris' assistant revealed himself to be the Sixth Doctor. The Master activated a machine to drain the life energy of Jago and Litefoot, before absorbing the Doctor's artron energy. The Doctor reversed the flow of the machine and the Master's life began to be drained. After the Doctor smashed his equipment, the Master slipped into his TARDIS and escaped. (AUDIO: Masterpiece)

Surviving in the universe
With his TARDIS still in the form of a clock, the Master attempted to steal Iris Wildthyme's body, (PROSE: The Scarlet Shadow) and was captured by the Sild. (PROSE: Harvest of Time)

Still looking a little "putrescent", (COMIC: The Abominable Showmen) the Decayed Master was greeted by a female incarnation of himself known as "Missy", who had developed a plan to form a band to hypnotise viewers of Battle of the Bands Beyond the Stars. His next incarnation, an incarnation possessing the body of a man named Bruce and an incarnation going by the name "Saxon" all joined in the plan, and the team spent "decades" practicing. (COMIC: The Five Masters) He played the electric guitar. After unveiling their presence to the Twelfth Doctor and Clara Oswald, (COMIC: The Abominable Showmen) the Masters prepared for their performance. However, the "Tremas" Master began to fight with Missy over the control of her device, believing that he alone could hold the universe in his grasp. The Decayed Master joined in the struggle, declaring that his future selves were idiots. The five were quickly disqualified, seemingly destroying them. (COMIC: The Five Masters)

First meeting with River Song
Posing as a Sun God, the Master forced the Therians to build a solar relais to prevent night from falling on Chasca Minor. After finding their worship boring, he ordered them to build him a tomb and seal him in it, as he entered a stasis chamber to await the Doctor. As he slept in the chamber, his tomb was removed from Chasca Minor and placed inside a museum on Chasca Major, where the Master was awakened by River Song and Luke Sullieman. Realising what had happened, the Master persuaded River to return him to Chasca Minor and retrieve his TARDIS in exchange for a chance to investigate the local ziqqurat.

Once they arrived on Chasca Minor, the Master began killing River's companions in order to both distract the Therians and pass the booby traps in the ziqqurat. Once the solar relais was deactivated, allowing night to fall, the Master still needed his solar capacitor to power his TARDIS, so agreed to help River escape from the planet in exchange for her help. However, River left the capacitor with the Therians, allowing them to revert their evolution and become people again, and abandoned the Master to their mercy. (AUDIO: Animal Instinct)

Fighting the Fourth Doctor
The Master plotted to capture the Z-battery that the Third Doctor left on Earth to repair his TARDIS during his exile. The Master's plan was to use the Z-radiation within the battery, combined with the O-radiation which permeated Oseidon, to create powerful ZO-radiation which the Master could use to renew himself. For his plot, the Master entered into an alliance with the Kraals, and created two robot duplicates; one to pose as Kraal Chief Scientist Tyngworg, (AUDIO: The Oseidon Adventure) and a second to search for genetically engineered alien worm in Derbyshire. In Derbyshire, he spent his time living underneath Hugh Spindleton's house. He offered Leela to the worm, but the worm didn't want to eat her. Knowing that the Doctor was near, he advanced his plans and wanted to capture him. He mused why the worm wanted the Doctor. He activated the worm by generating a lightning storm. (AUDIO: Trail of the White Worm)

The Master wanted to kill the Doctor with his TCE. He was arrested by Grimnal, but rescued by Leela, and they both went to Oseidon. The duplicate that stayed on Oseidon destroyed the one from Earth. The Doctor defeated the Master by using a Master android duplicate that he had constructed to kidnap the real Master, and take him away in his own TARDIS before his plan could be fulfilled. (AUDIO: The Oseidon Adventure)

The Master then posed as Inspector Efendi of the Intergalactic insurance agency so that he could find spaceships full of gold bullion. He then employed the Salonu to steal this gold, which attracted the attention of the Doctor and Leela to investigate. The Master then used the telepathic abilities of the Salonu to influence Leela into thinking that she was the Master's assassin and that he was the great Xoanon who desired the death of the Doctor. The Salonu Prime, with the help of the Doctor, noticed the Master's influence and undid the conditioning. (AUDIO: The Evil One)

Shandar of the Rocket Men invited the Master on his ship the Asteroid, where the Master saw the Doctor was Shandar's prisoner. When he confronted the Doctor, the Master used his Tissue Compression Eliminator on him, and apparently killed him, but he realised he had only destroyed a duplicate. The real Doctor was in fact pretending to be Oskin, and used that guise to bring down the force field around the ship, and used K9 Mark I to stall the Master's TARDIS once it had passed the force field so that the slaves on board the Asteroid could be freed. The Master overrode K9's tampering and kidnapped Leela after she had left the Doctor. (AUDIO: Requiem for the Rocket Men)

Charming the participants' owners with fine dining, the Master became the huntmaster of the Death Match, and used Leela as his champion. The Master said all that he was doing was reviving an old Gallifreyan past-time and sent the Doctor into the Game. He then set the endgame protocols which meant every contestant had an hour to live. Kastrella worked out that his plan was to wipe out the heads of major armies using his game to do this. He indulged her in killing all her rivals. After the Doctor reversed the command Matrix, he was to be killed as part of the endgame and would only live if he killed Kastrella. (AUDIO: Death Match)

The Master attempted to rend asunder the constellation of Mandus using a segment of the Key to Time, but was defeated by the Doctor. (PROSE: Cold Fusion) On Kendrax, the Master attempted to ally himself with the Daleks and the Cybermen, but his plan was foiled by Romana II. (PROSE: Special Occasions: 1. The Not-So-Sinister Sponge) He also entered a pact with the Embodiment of Gris, but found himself again bested by the Doctor. (PROSE: Cold Fusion)

Grasping at life
The Master was drawn to and became stranded on the planet Traken, the centre of the Traken Union, in a TARDIS configured into the sculpture-shaped Melkur. He plotted to take over the Source also located on the planet Traken, the power behind the Traken Union, and use it to restore himself. To this end, over a period of years, he won over Kassia, who later married Tremas and became a stepmother to Nyssa. His plans were thwarted by the Doctor and Adric when the Keeper of Traken summoned them, having sensed something of his machinations. With the help of Tremas and Nyssa, the Doctor removed the Master from the Source. However, with some of the Keeper's powers lingering, the Master was able to merge with Tremas, (TV: The Keeper of Traken) regenerating himself into a new body. (AUDIO: The Light at the End)

A new body at last
Having managed to replace his decaying Time Lord body with that of the Trakenite Tremas, the Master continued to face off against the Doctor. The circumstances of this stolen body's end vary across accounts. According to one account, the Master survived in this stolen form until he faced trial at the hands of the Dalek Prelature. (PROSE: The Eight Doctors) Another account indicated that the Master, having allied himself with the Tzun Confederacy, received a new regenerative cycle and achieved a new body before facing trial. (PROSE: First Frontier, The Novel of the Film) According to yet another account, however, the Master was stripped of his Trakenite form by the Warp Core. (AUDIO: Dust Breeding)

Reduced to decay
Now reduced to his decaying form, the Master collected four Krill eggs with the intention of awakening the Warp Core from its slumber and exhausting it, so that he could draw it into his TARDIS to be his slave. The Master then used a mask to disguise his deformity and followed the Warp Core as it arrived on Duchamp 331. Under the alias "Mr. Seta", the Master funded Madame Salvadori's trip to Duchamp 331.

There, the Master unleashed the Krill upon the passengers, hypnotising Salvadori's aide, Klemp, in the process. Revealing his true identity, the Master kept Salvadori alive, before encountering the Seventh Doctor. When the Core arrived, the Master tried to ally with it, but it dismissed him, leading him to ordering Klemp to kill Salvadori, but Klemp's loyalty was too strong, so the Master killed him. The Doctor escaped to his TARDIS, and attempted to gain control of the Warp Core through his TARDIS's telepathic circuits, while the Master used his TARDIS to fight of the Doctor's influence, and gain control of it. After it and the planet was destroyed, the Master was flung through time and space. (AUDIO: Dust Breeding)

The Doctor later made a deal with Death for the Master to have ten years of peace and sanity, at the end of which the Doctor had to kill him. To this end, Death transformed the Master into "John Smith", an ordinary physician on the colony world of Perfugium with no memory of his past. Smith was taken in by Wolstonecroft, and inherited his house when Wolstonecroft died, and became emotionally involved with Jacqueline Schaeffer. The Master remained active in Smith's subconscious, but was unable to influence the world around him.

At the end of the allotted time, the Doctor arrived to kill Smith, but strove to avoid fulfilling his side of the bargain. Death herself was present at these events, disguised as Smith's maid, and manipulated events so that the Master would become dominant once more. Her endgame was for Smith to make a decision that would ensure he remained in control; to kill Victor Schaeffer or allow Jacqueline to die by her husband's hand, but Smith was unable to kill Victor, (AUDIO: Master) and became the Master again as a result. (PROSE: The Tramp's Story)

The Deathworm Morphant
After being executed by the Dalek Prelature, the Master used a Deathworm Morphant to ensure his survival by possessing the body of San Francisco ambulance driver Bruce Gerhardt. The Master, knowing that this body would not last long, hatched a scheme to steal the body of the Eighth Doctor, only to fall into the Eye of Harmony. (TV: Doctor Who) As the Eye of Harmony tried to break him down into pure energy, the Master managed to ride the energy into a spare room, which the Doctor's TARDIS then ejected into the Time Vortex to protect the Doctor. (AUDIO: The Lifeboat and the Deathboat)

The Holder of the Glory
Nearing his ultimate destruction, the Master was rescued from the Vortex by a being named Esterath, the controller of the Glory, the focal point of the Omniverse. After gliding over the many realities throughout the Omniversal Spectrum for what he described as seeming like "centuries", the Master was resurrected into the body of a recently deceased vagrant on the streets of 2001 Brixton. Some weeks afterwards, due to a symbiotic link he had formed with the Doctor's TARDIS when it consumed part of his essence after he passed through the Eye of Harmony, the Master was transported onto the Moon during one of the Doctor's adventures. The Master subsequently used this link to trail the Doctor for some time without his enemy suspecting. The Doctor's companion Kroton, after becoming the controller of the Glory, cleansed the TARDIS of the Master's influence and placed the Master somewhere that he could not escape. The Master declared he would survive and return. (COMIC: The Glorious Dead)

Escape from the Eye
Once more trapped within the Eye of Harmony, the Master eventually escaped by influencing the dreams of Edward Grainger to unravel the Doctor's timeline, by killing Edward Grainger whilst he was an infant in 1906. (PROSE: Forgotten) Though his Morphant form had long since sublimated to a gaseous state, the Master was still capable of using its powers to possess new bodies. (AUDIO: Mastermind) However, while in the body of Sir George Steer, he was stopped by an older Edward Grainger from 2006 and Violet after being hit with a rolling pin and being removed from George. (PROSE: Prologue to The Centenarian) The Master then managed to evade the Doctor's detection, and possessed the body of a human named Richard. (PROSE: Forgotten)

After possessing Richard, the Master killed Violet out of revenge. However, the Master discovered his possession had caused the host body to decay at an accelerated rate, so he was forced to steal more bodies to prolong his survival. Realising that the First World War was rapidly approaching, the Master decided to migrate to America to avoid the conflict and boarded a ship to go there in 1912. Ironically, he had boarded the RMS Titanic, unaware of its eventual fate, and escaped in a lifeboat when it sank.

Arriving in New York City, the Master took possession of a member of the Hudson Dusters, quickly becoming the leader of the gang and calling himself "Don Maestro". After twenty years of living in his current body, he occupied the body of his host's son, Michael, and moved to Las Vegas where he owned a casino. He accumulated money to fund experiments towards the elongation of the lifespan of his host body. Fearing the eventual decay of his body, the Master used his money to buy a penthouse to isolate himself from infection. After years living in isolation, his host's son confronted him with the knowledge that he had possessed both his father and his grandfather in some way. He then trapped the Master in the penthouse.

After UNIT were alerted to the presence of penthouse, they discovered the Master in a comatose state. He was imprisoned in the UNIT Vault, awakening every five years for one hour, before returning to a coma. After fifteen years living in the Vault, the Master awoke for a third time and was interrogated by UNIT officers Ruth Matheson and Charlie Sato. However, he managed to hypnotise both of them and escape his imprisonment. Discovering that UNIT had recovered his TARDIS from a sealed tomb in the Valley of the Kings, he used it to escape from the Vault. (AUDIO: Mastermind)

The final end
The Master contacted Earth from the planet Glox from his TARDIS, disguised as a sound studio. As he told humans about how he brought Glox's civilisation to its ruin, he implanted into the listener a telepathic order. When the Doctor discovered his hideout, the Master ordered one of the listeners to connect telepathically to his TARDIS and become a replica of him; he then sent him out to face the security of the place and be killed in his place, while he escaped. (AUDIO: I Am The Master)

After arriving on the planet Parrak in search of the tomb of one of Rassilon's leading engineers named Artron, the Master used his TARDIS to extract all water on the planet's surface throughout its history to use as an incentive for his workforce to keep obeying his rule. His overall plan was to use the Tomb of Artron to revive himself, using the unlimited regenerative energy Artron had discovered on Kolstan.

Nearing the end of his life and getting increasingly more desperate, the Master allied himself with the Eleven, with the intention of betraying him once the tomb was opened. However, the Eleven had used the Master to gain access to Artron's matrix brain, which he used to lead the Ravenous to the Doctor. Upon the Ravenous' arrival, they took great interest in the Master, believing that the flavour of all of his lives was superior to that of the Doctor's. The Ravenous feasted on him and the Master finally died, his body being left on Parrak. (AUDIO: Planet of Dust)

However, when the Celestial Intervention Agency requested the "Bruce" Master, the "War" Master and Missy to return Artron's brain print to Parrak to help fuel the technology to grant new regenerative cycles to Time Lords, the Masters used the technology for themselves as part of the deal, and resurrected the corpse of the dead Master, giving him a new body and regeneration cycle in the process. (AUDIO: Day of the Master)

Post-mortem
While his essence had eventually escaped through the Time Vortex, (COMIC: The Glorious Dead; PROSE: Forgotten) an "echo" of the Master remained imprisoned inside the Doctor's TARDIS. In the singularity of the Eye of Harmony, the Master commanded infinite power, but could only wield it from within the confines of the Eye. The Master became externally perceived as a Great Black Eye beyond time, which frightened the Time Lords of Ulysses' era, was worshipped as a god in the post-War Kingdom of Beasts, (PROSE: The Gallifrey Chronicles) and poisoned the Eighth Doctor's second heart. (PROSE: The Adventuress of Henrietta Street)

Shortly after his defeat, the Master laid a final trap for the Doctor, leaving a crystalline structure on the Eye that would give the Doctor amnesia. However, the Doctor was subconsciously guided by Rassilon to recover his memories by visiting his previous seven selves. (PROSE: The Eight Doctors) Whilst exploring the Eighth Doctor's TARDIS, River Song thought she heard an American screaming from within the walls. (GAME: The Eternity Clock)

During his imprisonment, the Doctor came to speak to the Master where he dwelled in a room with his face on a screen. The Master spoke to the Doctor in the room on several occasions, telling him that he was an "old friend". After the defeat of the Council of Eight, the Doctor spoke to him about the death of Miranda Dawkins and if it was worth the cost of him saving the universe. (PROSE: Sometime Never...)

The Master later appeared to the Doctor within a mirror in the TARDIS, where he asked the Doctor what was going on inside his head. The Doctor was unable to answer the question before the image in the mirror returned to that of the Doctor's reflection. (PROSE: The Deadstone Memorial)

The Master had a longer conversation with the Doctor from within the Eye of Harmony. The Master showed the Doctor a vision of Marnal's investigation of the Shoal. When the Doctor pressed him about what happened to Gallifrey, the Master teased him with offers to bring it back and to return his memories. The Master then became angry over the circumstances of his imprisonment. He threatened to use all of his power to detonate a fusion device and have his revenge. The Doctor sealed up the Eye of Harmony before he had the chance to carry out his plan. (PROSE: The Gallifrey Chronicles)

During the War in Heaven, Father Kreiner killed and decapitated either the original Master or a clone created as part of the Time Lords' War-time hatchling projects. (PROSE: Interference - Book One)

Alternate timelines
Discovering that the Celestial Intervention Agency were gathering illegal Vess weapons, the Master blackmailed their agent, Straxus, into handing over a conceptual bomb. The Master then visited Bob Dovie and, after killing his family, planted the device into his head. When Dovie saw the inside of the Doctor's TARDIS, his refusal to believe in it caused the Doctor's TARDIS to explode, causing its timeline to begin to collapse. With the Doctor's timeline collapsing along with the TARDIS's, the Doctor's first eight incarnations joined forces to avert the detonation of the bomb, before the First Doctor erased the events from history. (AUDIO: The Light at the End)

Personality
While he originally approached a situation with youthful arrogance, (AUDIO: The Two Masters) the Decayed Master preoccupied his time with finding a way to regenerate following his disfigurement and the loss of his own ability to regenerate forcing him to face his imminent death. With his mobility and capabilities of camouflage decreased, he was often forced to hide his involvement in his plans until the very moment victory was within his grasp. (TV: The Deadly Assassin, The Keeper of Traken)

Blaming him for his predicament, (PROSE: The Dead Travel Fast) the Decayed Master felt a stronger hatred towards the Doctor than before, specifically guiding the Fourth Doctor back to Gallifrey so he could be framed for the President's assassination and executed in disgrace, (TV: The Deadly Assassin) wished to personally kill a companion of the Doctor, (AUDIO: The Two Masters) and once hatched a plan that would have destroyed all the Doctors and unravelled the Web of Time simply for his revenge against the Doctor. (AUDIO: The Light at the End) He also disliked being compared to the Doctor. (AUDIO: Animal Instinct) Despite the animosity, the Master was able to have a civil conversation with the Doctor when it suited him, (AUDIO: Death Match) and showed shades of bitterness when he learned River Song was the Doctor's wife. (AUDIO: Animal Instinct)

While he claimed that nothing he ever did "[was] ever pointless", (AUDIO: The Light at the End) and that he only killed for "power", (AUDIO: The Two Masters) this incarnation of the Master seemed more comfortable with killing people just for the sake of it, (AUDIO: The Light at the End) showing a sadistic pleasure when he resorted to killing, (TV: The Keeper of Traken) and even destroyed the planet Raskalar for amusement. (AUDIO: Death Match) However, the Seventh Doctor recalled how the Decayed Master was "generally a serious sort", remembering how he was "cold and cruel." (AUDIO: The Two Masters)

Leela claimed that this incarnation was "raw" and "honest", as he "did not seem to hide [himself] away" or "disguise [his] hate". (AUDIO: The Devil You Know)

Skills
In his degenerated state, the Master's telepathic capabilities and willpower grew stronger, with the Master proclaiming that "only [his] hate [kept him] alive". He was able to launch a telepathic message to the Doctor from Gallifrey to the Doctor's TARDIS, (TV: The Deadly Assassin) and, once he became the Keeper of Traken, the Master forced Tremas to kill Neman through sheer willpower, and also paralysed the Doctor to make him watch. (TV: The Keeper of Traken) However, he was unable to hypnotise the Proto-Time Lord River Song. (AUDIO: Animal Instinct)

Meticulous in his schemes, the Decayed Master planned for every imaginable obstacle and put in place a counter for it. (AUDIO: Trail of the White Worm, The Oseidon Adventure) He was willing to be patient with his plans, waiting inside his TARDIS for years to slowly seduce Kassia. (TV: The Keeper of Traken)

Despite his frail body, the Decayed Master was an admirable fighter, able to trade equal blows with the Fourth Doctor, (TV: The Deadly Assassin) stand his ground in a brawl with his other incarnations, (COMIC: The Five Masters) and face off with Bram Stoker while suffering Artron decay. (PROSE: The Dead Travel Fast)

Appearance
The Master came to resemble a deformed corpse, (TV: The Deadly Assassin) with brown eyes. (TV: The Keeper of Traken) However, after absorbing some energy from the Eye of Harmony, (TV: The Deadly Assassin) he became less "putrescent". (AUDIO: Trail of the White Worm) With his decayed body, the Master would experience almost unendurable pain. (AUDIO: The Two Masters)

By the time of his death, the Master had continued to rot away, and had begun wearing a hooded cloak to conceal himself and was covered in bandages, with the Eleven comparing him to a mummy. (AUDIO: Planet of Dust)

He was described by Spandrell as being "emaciated", (TV: The Deadly Assassin) with Bob Dovie describing him as looking "burned." (AUDIO: The Light at the End) The Fourth Doctor described him as "a cowled cadaver", (AUDIO: Requiem for the Rocket Men) while River Song described him as the "crispy-looking Master". (AUDIO: Animal Instinct) Missy remembered her wraith-like thirteenth incarnation as being "the Yucky One". (PROSE: Meet Missy!)

Clothing
To hide his disfigurement, the Master took to wearing a rotting hooded cloak. (TV: The Deadly Assassin) Whilst in Victorian London, he wore a mask in public to conceal his decaying appearance, and also used a cane to aid his frail body. (AUDIO: Maurice) While claiming to be "Mr. Seta", the Master wore an ornate, bejewelled golden mask. (AUDIO: Dust Breeding)

When trapped on Earth in the 20th century, the Master wore whatever his hosts wore normally, to avoid detection. He discovered each body he possessed would inevitably revert to a decayed appearance, despite his best efforts to prolong them. (AUDIO: Mastermind)

The Master was briefly given a perception filter by Missy to hide his decaying appearance making him appear, in his own words, "handsome". He deactivated it at the suggestion of, prompting Kitty to scream. (AUDIO: Masterful)

Other references
During his initial confrontation with the "Tremas" Master, the Fourth Doctor saw the Decayed Master as he appeared on Gallifrey calling out to him amongst other enemies before he fell from the Pharos Project, resulting in his regeneration into the Fifth Doctor. (TV: Logopolis)

Shaw Taylor acknowledged the Decayed Master as the second of the Master's three known "regenerations", between the "UNIT era" Master and the "Tremas" Master. (TV: Police 5: The Master)

As recorded in the Masterplan Journal left behind by Missy, the "Saxon" Master had deemed there to be no room for "Mister Charcoal Grill" when he carved the image of his current self along with "Beardy One", "Beardy Two" and "Wizard of Oz" into Mount Rushmore during the Year That Never Was. (PROSE: The Secret Diary of the Master)

Whilst trapped in the Matrix by the Spy Master, the Thirteenth Doctor remembered, amongst many others, the Decayed Master as he appeared on Gallifrey in order to break out of the computer system. (TV: The Timeless Children, TV: The Deadly Assassin)

Behind the scenes

 * During the final shot of the Master in The Deadly Assassin, the Master was considerably less decayed than he was during the rest of the story. It was apparently the intention that, having received enough energy from the Eye of Harmony, the Master was beginning to regenerate. However, this scene was omitted from the novelisation and was thus forgotten by the time of the production of The Keeper of Traken. The Master's apparently healthier appearance in the latter would be explained in Trail of the White Worm, which establishes that the Master partially healed himself in The Deadly Assassin.
 * It is unclear whether this incarnation of the Master is the same incarnation portrayed by Roger Delgado or a subsequent incarnation. While earlier accounts tended to favour the idea that the incarnation was simply a significantly deformed version of the Twelfth Master (PROSE: Legacy of the Daleks), more recent accounts have favoured the idea that the incarnation is distinct from his predecessor. (AUDIO: The Two Masters, Masterful)
 * The identity of "Weng-Chiang" in The Talons of Weng-Chiang was initially intended to be that of the Master rather than Magnus Greel. These plans were abandoned when producer Philip Hinchcliffe decided that he didn't want to have the Master revealed as "the secret villain" so soon after The Deadly Assassin. Alan Barnes has pointed out several clues in the story that still point to the original plan: the "time cabinet" and Greel referring to Leela as "the first morsel to feed my regeneration". (DWM 475)
 * The behind the scenes book A Celebration (1983) had a feature entitled The Two Regenerations of the Master, also referring to The Two Faces of the Master, namely the Delgado Master and the Ainley Master. From a production standpoint, Delgado was identified as the "first Master", whilst Ainley was designated as both the "second incarnation" and the "second regeneration" of the Master, though the feature does also acknowledge the interim "twelfth and final regeneration" portrayed by Peter Pratt and Geoffrey Beevers.
 * When initially pitching The Curse of Fatal Death, Steven Moffat deliberately linked his story treatment to the gap between " running away at the end of Frontier in Space and Peter Pratt's cowled and rotted-faced incarnation turning up in The Deadly Assassin." This was eventually abandoned, with instead walking off into the distance with the freshly regenerated Doctor.
 * His first audio appearance in Dust Breeding was originally intended to be a story starring the "Tremas" Master. The Trakenite body being stripped away was written in due to an inability to negotiate a suitable contract with Anthony Ainley. (REF: The Audio Scripts: Volume Two)
 * Amongst the names used by the Master in the parallel universe of Exile are Peter and Geoffrey, a reference to Peter Pratt and Geoffrey Beevers.
 * Battles in Time released a card for The Deadly Assassin Master, distinguished from other incarnations by the title "The Master (Emaciated Form)".
 * A figurine of the "Emaciated Master" as seen in The Deadly Assassin was released with issue 49 of Doctor Who: Figurine Collection.
 * In the story of Doctor Who: Legacy, John Simm's retrieves the "Decaying Master", appearing as he did in The Deadly Assassin, from the collapsing reality as he assembles his other incarnations.
 * In The Planet of Dust & Other Stories, Beevers gave an audio reading of the Doctor Who Annuals' story The Creation of Camelot, which featured a "generic" bearded Master, raising the possibility of this Master as a pre-decay version of the Thirteenth Master.
 * "The Decaying Master" was the internal name used to distinguish the incarnation of the Master portrayed by Geoffrey Beevers in Masterful, as revealed in the behind-the-scenes track for its release.
 * Geoffrey Beevers also played the Master of the Warrior's universe in the Doctor of War, however, it is not made clear if this Master is intended to be an analogue to the Decayed Master or some other incarnation. The Warrior claims he has not met him before, though he was also suffering from memory loss, and it is not clear if the incarnation that fought with him in the Time War before the trial was the same one.

FASA
The Doctor Who Role Playing Game by FASA, which admits to taking liberties with the source material in its opening pages, gives a rundown of the Master's first thirteen incarnations in "The Master" supplement book, which was similar to (but not entirely consistent with) the in-universe biography given for the Master in FASA's own CIA File Extracts.

According to the book, the Master could control the form of his incarnations, and frequently used the same face. The Master appeared in a constant, bearded aristocrat form until his fifth incarnation became a renegade and regenerated into an "average" clean-shaven form, becoming known as the Monk in his sixth and seventh incarnations. When the Monk was exposed, the Master chose a new disguise and assumed a bearded appearance similar but distinct to his initial form. Though the Master's thirteenth incarnation retained his familiar form, it changed when injuries brought him to the brink of death without the ability to regenerate. Whilst the Master's iron will refused to accept annihilation, his body began to decay, becoming hideous and skeletal with his face a noseless, grinning death's head cloaked in shards of rotting flesh. The Master's voice, once resonant and commanding, was reduced to a hissing rasp, and he retained little control over his dying frame, which was covered with a black cloak and hood. Even so, his intellect remained undimmed and he eventually seized the body of Tremas, which he wore through his fourteenth and fifteenth incarnations.