Rassilon

Rassilon was perhaps the single greatest figure in Time Lord history. Certainly he was the most important figure in the official histories of Gallifrey, and was generally considered the first Time Lord, though some believe that distinction belonged to Omega. He is the founder of Time Lord civilization.

Most of Time Lord society hails him as a hero. However some contend that Rassilon was in fact a corrupted megalomaniac who tried to murder his friend Omega (falsely believing he succeeded), and stole his invention to build Time Lord society. In his only two appearances in the TV series he's portrayed as an intimidating and unforgiving person, but in his first appearance also resented corruption and was good at detecting it. Ironically by his second appearance he was entirely guilty of this trait.

Achievements
Prior to the creation of the Time Lords, Rassilon headed the Prydonian Chapter. With Omega, he planned to make the star Qqaba go supernova. The energy released would enable the Gallifreyan race to travel through time, to become Time Lords. A saboteur from the future known as Fenris, caused the death of Omega (as the official story goes) but the time controller on Fenris' belt proved useful to him. (DWM: Star Death)

Rassilon ended the Dark Time and deposed the matriarchal society of the Pythia, in retaliation for which the Pythia cursed her enemies and made Gallifreyans sterile (NA: Cat's Cradle: Time's Crucible).

He brought the Eye of Harmony, the singularity of a black hole, to Gallifrey where it rested underneath the Panopticon (DW: The Deadly Assassin). He invented TARDISes, Gallifrey's transduction barrier, Looms and the De-mat Gun. Time Lord history states the traditions of Time Lord society originated with Rassilon, including principles of non-intervention (which may or may not be the same as the policy of non-interference) after he had a nightmarish vision of a dictatorial, imperialistic Gallifrey (DWM: The Final Chapter). The Time Lords themselves may have been created by Rassilon: the genetic link that enabled Time Lords to travel through time without ill effects was known as the Rassilon Imprimatur, and it is implied that many physiological traits of Time Lords, including the ability to regenerate, were his creations. (BFA: Zagreus)

Using temporal technology, Rassilon studied the future where he learned of the Divergence, a race that would eclipse his own within 10,000 millennia. Fearing this potential future, he created a self-replicating, biogenic molecule which he sent back in time to seed all habitable planets in Gallifrey's galaxy. This ensured that all intelligent life evolved in the form of the Gallifreyans who trapped the Divergents in their own timeline, which Rassilon sealed into a time loop.

He despised the existence of the Vampires and began a purge to wipe them out of existence. (BFA: Zagreus) This took the form of the war against the Great Vampires. Following the war, he believed every one of the Great Vampires in the universe dead. (DW: State of Decay)

Artifacts
A great many important Gallifreyan artefacts bear his name, including the Sash of Rassilon, Rod of Rassilon, Coronet of Rassilon, Harp of Rassilon, Ring of Rassilon, Great Key of Rassilon and the Loom of Rassilon. The symbol used as a mark of Time Lord authority is known as the Seal of Rassilon; it appears as a motif in many Time Lord designs.

Supposed Death
Many rumours also surround Rassilon's death (or lack thereof). One stated that the Time Lords had revolted against him and imprisoned him in the Dark Tower in the Death Zone. Borusa, Lord President and the Doctor's former mentor, believed that Rassilon had discovered a form of true immortality beyond the regenerations known to Time Lords. Borusa uncovered and used several artefacts from the Dark Times, including the Coronet of Rassilon (a mind control device) and the Game of Rassilon. The Doctor discovered the truth when his mentor Borusa used him to try and discover Rassilon's secret: Rassilon did indeed discover immortality, but realised it was too dangerous a secret to share. Borusa was condemned to immortality as a living statue, imprisoned immobile in Rassilon's tomb. (DW: The Five Doctors)

Other accounts show that Rassilon lived on inside his creation, the Matrix (BFA: Neverland, Zagreus), with other beings. Together they called themselves the High Evolutionaries and would have some involvement in the affairs of the universe. (DWM: The Tides of Time, The Final Chapter) Rassilon and the other Evolutionaries once hired the Threshold to stop a Dalek plan to invade other realities. (DWM: Fire and Brimstone) Further accounts suggest that he became trapped within the Divergent Universe. (BFA: The Next Life)

Return
During the Last Great Time War, Rassilon was again the Lord President of the Time Lords after replacing President Romana. Now incredibly ruthless, and determined to avoid death, on the last day of the Last Great Time War, he intended to spare Gallifrey from destruction by using the End of Time to turn the Time Lords into beings of pure consciousness, but at the cost of the rest of creation. His plan was defeated by the Eighth Doctor, who destroyed Gallifrey by using The Moment, but apparently set the planet on fire in the process. Before the Doctor destroyed Gallifrey, however, Rassilon attempted to break Gallifrey out of the time lock that blocked the Last Great Time War from temporal manipulation, but was defeated by the Tenth Doctor and was badly hurt by a last attack from a vengeful Master. Rassilon, the Time Lords, and Gallifrey, were all sent directly back to the Time War, along with the Master. (DW: The End of Time)

It is possible that the Time Lords resurrected the Rassilon of old that his wisdom might aid them in the War effort, similarly to how they resurrected the Master after he fell into the Eye of Harmony.

Behind the scenes

 * Rassilon was first mentioned in DW: The Deadly Assassin. Prior to this, evidence pointed to Omega (who had appeared in DW: The Three Doctors) as the founder of Time Lord society. A Doctor Who Weekly comics story, Star Death attempted to reconcile this by having Rassilon and Omega working together. A few subsequent televised Doctor Who stories, such as Remembrance of the Daleks and Silver Nemesis, also took this approach. Remembrance's novelisation by Ben Aaronovitch, dipping into the "Cartmel Masterplan" explained that Omega, Rassilon and a third historical figure, the Other worked together in enabling the Gallifreyans to have time travel. The Infinity Doctors by Lance Parkin, which features Omega, states that the Time Lords had not three but six important central figures, the names of three remain unknown. The spin-off nature of the Other and the remaining three figures casts doubt on their canonicity.
 * Though his Doctor Who Monthly appearances had shown him as a rather foreboding character, and The End of Time painted him as a villain, the much earlier The Five Doctors outright stated that Rassilon might have a sinister agenda, possibly since he knew the secret of immortality, he grew power-hungry and became something of a hypocrite when it came to his "Watch, and Not Interfere" policy thus, he began to corrupt the High Council of Time Lords.
 * It should be noted that, at no point in The End of Time, is it explicitly stated that the Rassilon in that story is the same individual as depicted previously. Just as the monarchs of many nations and Popes often share the names of previous monarchs and Popes, it's possible this Rassilon took his name from the Rassilon of old. Though this is unlikely as the Time Lords would probably not allow anyone else to take the name of 'Rassilon', and it is stated by Russel T. Davies in The Writer's Tale: The Final Chapter that it is indeed the same Rassilon as old, brought back in the same manner as the Master.