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.

Achievements
Previous 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 (or so the Gallifreyans believed) but the time controller on Fenris' belt proved useful to him. (DWM: Star Death)

Rassillon 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 (source?) Gallifrey's transduction barrier, Looms and the De-mat Gun. History said of the traditions of Time Lord society originated with Rassilon, though notably not their policy of non-interference. 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 learnt of the Divergence a race that would eclipse his own within 10,000 millennia. Fearing this potential future, he created a sent self-replicating, biogenic molecule which he sent back in time to seed all habitable planets in Gallifrey's galaxy, which ensured all intelligent life evolved in the form of the Gallifreyans which trapped the Divergents in their own timeline, which Rassilon sealed into a time loop. Despising 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.

Death
Many rumours also surround Rassilon's death. 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)

Behind the Scenes

 * Mention of Rassilon first in The Deadly Assassin. Prior to which all evidence pointed to Omega, who had appeared in 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 and a few subsequent televised Doctor Who stories, Remembrance of the Daleks and Silver Nemesis have taken 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.
 * Though his Doctor Who Monthly appearances had shown him as a rather foreboding character, The Five Doctors first outright stated that Rassilon might have a sinister agenda.