The Master (The Creation of Camelot)

A version of the Master once posed as the Merlin in Camelot, but his plans to throw England's history off the rails were foiled by the Fifth Doctor and Tegan Jovanka. He would subsequently have two encounters with the Sixth Doctor and Peri Brown, posing, during the first, as the Sentinel of Quan.

As "the Merlin"
The Master arrived in Camelot just after the coronation of King Arthur, and became the new Merlin after the old one died. He arranged the birth of a son, Mordred, to Arthur's resentful half-sister Morgan so that Mordred would grow up to kill Arthur at the battle of Camlan; Arthur foresaw this and despatched "the Merlin" to Lothian to kill the baby, but the Master spirited him away to raise him in secret.

He also began to pass secret information to the Saxons and encourage them to start breaching Arthur's borders much earlier than they were supposed to do, in an effort to destabilise history, and weakened Arthur's court by turning the knights against each other. He often "disappeared for large spans of time", skipping ahead through Camelot's timeline thanks to his TARDIS, but in the gulliple Arthur's mind, this only added to the mystique of his "necromancer".

By chance, however, the Fifth Doctor and Tegan Jovanka arrived in the same time-zone and realised what was going on when Arthur showed familiarity with the word "TARDIS", having heard his Merlin mention it. Confronted by the Doctor and Tegan in front of Arthur, he seemingly gave in when he was ordered to go back to his chambers to await trial. The Doctor realised too late that the Merlin's "rooms" were actually a disguise for his TARDIS, and he was able to escape. To counteract the dissent the Master had sown in King Arthur's court, the Doctor suggested the foundation of the order of the Knights of the Round Table. (PROSE: The Creation of Camelot)

The Fellowship of Quan
The Master discovered the existence of a powerful, dormant robot called Quan on the ruined mining planet Tuven III. He killed and replaced the Sentinel of Quan, the leader of the cult-like Fellowship of Quan descended from the original miners who dwelled in the underground tunnels of the planet and guarded the secrets of the old machines, but they were unable to translate a key inscription, as was the Master himself.

Guessing that the Doctor would be better able to translate it, he used another one of the old machines to create a localised spatial vortex which drew the Sixth Doctor's TARDIS off-course. Captured and taken before the "Sentinel", the Doctor and Peri were led to the chamber where Quan was in stasis, where the Master revealed his true identity to the two. However, instead of the docile machine the Master had expected, Quan was a fully independent entity who made its own choices. The Doctor convinced him that the Master was not to be trusted, and Quan promised to instead help the Fellowship make the planet habitable again. With the Master's true identity and motives revealed, the cultists began to crowd around the Master with murderous intent. To Peri's shock, the Doctor refuses to help the Master and simply leads her back to the TARDIS despite the Master's pleas. (PROSE: The Fellowship of Quan)

The radio plot
The Master next travelled to London in the 20th century, at a point when radios were becoming widespread in the population, making his lair in the Post Office Tower by hypnotising a group of workmen carrying out repairs. He then used the Tower to take over the air, broadcasting his hypnotic abilities throughout the city at set times, with the goal of gathering a hypnotised crowd at the State Opening of Parliament to lynch the politicians and the British Royal family and make himself the dictator of the British Isles. However, he made the mistake of testing his hypnosis a few days early by getting a small crowd to stand around the House of Parliament to await instructions for almost an hour. Though this trial run went smoothly, it attracted the attention of the Chief Inspector, who called in the Sixth Doctor and Peri Brown for help.

The Doctor quickly figured out the Master was behind the plot, and hunted him down to the Post Office Tower, where he recognised the Master on sight. Confronted with the armed forces brought by the Doctor and the Chief Inspector, the Master quickly admitted defeat, on the condition that he personally switch off the central hypnosis device. In the process, he also pressed the controls of his invisible TARDIS and vanished, flashing a malicious smile at Peri. (PROSE: The Radio Waves)

Appearance
In all three of his documented encounters with the Doctor, the Master inhabited a body with pale skin, prominent cheekbones, a neatly-trimmed black goatee, and voluminous hair with a widow's peak. (PROSE: The Creation of Camelot, The Fellowship of Quan, The Radio Waves) His swept-back hair and pointed beard were what allowed Peri Brown to recognise him as the Master. (PROSE: The Fellowship of Quan)

While dressed in period-appropriate noble garb as the Merlin of Camelot, he wore a "long cloak of black velvet with a high stiff collar" whose claps was carved to resemble a skull, with a "silver robe" underneath. (PROSE: The Creation of Camelot) On Tuven III, as the Sentinel of Quan, he wore a dark gray shirt and trousers with a light-green hooded cloak over them. He kept the hood up to disguise his face from the Brothers of Quan so that they would not realise he was not the original Sentinel, whose voice he mimicked using a voice-filter device hidden on his person. (PROSE: The Fellowship of Quan)

Behind the scenes
This Master was termed a "generic" Master by The Essential Doctor Who #4's feature on The Radio Waves. With World Distributors, who published the Doctor Who Annuals, having failed to secure the rights to the likeness of Anthony Ainley, this unique design, resembling a blend between the two main televised Masters up to that date (Ainley and Roger Delgado) but not legally recognisable as either, was created instead. One of the illustrations in The Creation of Camelot seems to have been inspired by a drawing of Batman villain Ra's al Ghul in the story The Demon Lives Again!, first printed in 1972's Batman #244, most evident in the pose and outfit design.

The Creation of Camelot had likely been written on the assumption that the Master was Anthony Ainley's version and gave no explanation for the Master's manifestly different appearance, leaving this incarnation's place in the Master's overall timeline up to reader interpretation. Notably, in The Planet of Dust & Other Stories, an audio reading of the story was provided by Geoffrey Beevers, the portrayer of the Decayed Master, raising the possibility of the "Merlin" Master as a pre-decay version of the Thirteenth Master.