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Doctors: Fought the Eighth Doctor, War Doctor and the Tenth Doctor; aided the First, Second, Third and Fifth Doctors in resolving a crisis; confronted holograms based on the Fifth, Sixth and Seventh Doctors when he tried to take control of the Eighth Doctor.
Companions: Fought Charley Pollard, C’Rizz, Leela, K9, 2nd Romana, Cinder and Wilfred Mott; peripherally encountered Susan, Sarah Jane Smith, Turlough, Tegan Jovanka, The Brigadier and Donna Noble but did not address them directly (Met The Brigadier again, but on that occasion The Brigadier was a hologram created by the evil side of the TARDIS), available evidence suggests an indirect encounter with Wilfred Mott and Donna Noble. .
According to Time Lord legend, Rassilon was one of the three Founders of Time Lord society in the distant past, along with Omega and the mysterious individual known only as ‘the Other’. With the aid of Omega and the Other, Rassilon aided in the reformation of Time Lord society following the departure of the Pythia and the sterilisation of all Gallifrey, and also oversaw the creation of the Eye of Harmony in which Omega was lost (Later accounts speculate that Rassilon engineered Omega’s death so he wouldn’t have to share the glory). After the Other sacrificed himself to restore children to Gallifrey via the creation of the genetic looms, Rassilon led Gallifrey as its first President; officially, he was a wise, benevolent ruler, but some accounts paint him as a greedy despot who seized power after the deaths of Omega and the Other. Despite these contradictions, Rassilon’s contributions to Gallifreyian society are immense; the Rassilon Imprimatur, the symbiotic nuclei that allows Time Lords to safely travel through time in their TARDISes, is named after him, as are various Time Lord relics such as Sash of Rassilon, the Rod or Great Key of Rassilon, the Crown of Rassilon, the Coronet of Rassilon and the Harp of Rassilon, each relic having purposes beyond the purely ceremonial.
Rassilon is also credited with the creation of the timescoop (A device that can remove individuals from their allocated locations in space and time and placing them in a location of the operator’s choice), the living metal Validium, and the transduction barriers that defend Gallifrey, although many of these may be propaganda rather than fact. What is known is that Rassilon’s Tomb, known as the Dark Tower, stands at the centre of the Death Zone on Gallifrey, an area where Time Lords used to deposit various beings that had been captured by timescoop and pitted against each other for the amusement of Gallifrey as a whole. Some sources state that Rassilon shut the games down, but others say that Rassilon caused the games in the first place and was deposed by the Time Lords who objected to his rule. It was also rumoured among some that Rassilon had learned the secret of immortality, and was still alive in the Tower, sleeping and waiting to offer immortality to worthy Time Lords who braved his tests…
Eventually, this belief posed trouble for The Doctor when his old teacher, Borusa, was driven insane by the pressures of ruling Gallifrey, and became convinced that only he was intelligent enough to rule the Time Lords. Seeking the path to true immortality, Borusa used the timescoop to bring together the only people who he believed capable of passing Rassilon’s tests; the Fifth Doctor and his past four incarnations (Along with their companions Susan, Sarah Jane Smith, Turlough, Tegan Jovanka, and The Brigadier), although the Fourth Doctor and Romana were stuck in a Time Eddy due to a flaw in the timescoop. While the first three Doctors, accompanied by Tegan, The Brigadier and Sarah respectively, tackled the various perils of the Death Zone, Turlough and Susan were forced to stay behind in the TARDIS while the Fifth Doctor investigated events in the Gallifreyian High Council. Although the three Doctors made it to Rassilon’s Tomb in the Dark Tower, where they were able to deactivate the Tower’s defences and bring the TARDIS to them, Borusa then arrived to claim his prize, having taken control of the Fifth Doctor’s mind using the Coronet of Rassilon to emphasise his will. Although Borusa immobilised the companions, the First, Second and Third Doctors linked their minds and managed to free the Fifth from Borusa’s control. Rassilon was awakened at this point and, on the advice of the First Doctor, concluded that Borusa was ‘worthy’ of immortality; thanks to a riddle on the tomb, the First had deduced that the whole immortality story had been a trap by Rassilon to get rid of potentially dangerous Time Lords. All Borusa got for his trouble was an eternity trapped as a living statue, Rassilon then freeing the Fourth Doctor from the Time Eddy before restoring the First, Second and Third to their proper places in time.
Although he made a brief appearance in “Blood Harvest” when a regenade Gallifreyian security council attempted to free Borusa – only for Borusa to reveal that he had abandoned his old ways and side with The Doctor in defeating the group before he departed to another plane of existence , Rassilon eventually returned in “The Eight Doctors” when, from behind the scenes, Rassilon contacted the Eighth Doctor, currently missing his memories following a trap planted by The Master, and sent him through Time to aid his other selves, his lost memories being restored through telepathic contact with his past incarnations. Thanks to the Eighth Doctor, the First acknowledged that he had to be better than the Time Lords he’d abandoned, the Second gave his freedom to save those involved in the War Games, the Third was able to fight off an attack from The Master, the Fourth was saved from death by Vampires, the Fifth survived an attack from an old foe armed with a timescoop, the Sixth’s trial had an official enquiry, and the Seventh was saved from one of the Giant Spiders of Metebelis Three. The Doctor was honoured to serve Rassilon, and Rassilon, pleased with The Doctor’s actions, watched him as he saved Sam Jones from drug dealers and allowed her to join him in his travels.
Rassilon’s return to The Doctor was by no means smooth; at the time, The Doctor, along with former companion Romana, was dealing with the consequences of his attempts to save the life of his new companion Charley Pollard, even though history as a whole recorded that she should have died on the airship R101 had The Doctor not intervened. In “Neverland“, when The Doctor and Charley were caught by the Time Lords and The Doctor put on trial for the crime, it was revealed that, according to legend, Rassilon’s creation of the Eye of Harmony created one continuous history, but, as every action has its equal and opposite reaction, the creation of positive Time created a mirror universe of anti-Time, a force as destructive to causality as anti-matter is to positive matter, the anti-time realm being a chaotic universe with no past, present or future; just an endless, meaningless, ever-changing Now. This universe of anti-time was as damaging to normal Time as anti-matter was to matter (See “Omega“), and the damage to Time caused by The Doctor saving Charley was causing anti-Time to seep through.
Travelling into the anti-time universe, The Doctor, Romana and Charley encountered an old man who seemed to be Rassilon, who had entered anti-Time to battle the legendary monster known as Zagreus, but had been trapped after Zagreus’s defeat and forced to wait in his Zero Cabinet for aid. However, it was revealed to be nothing more than a shame; Rassilon’s mind did still live in the Matrix, but the Zero Cabinet was actually an anti-time bomb, which, when detonated, would shatter the centre of the web of time and contaminate the entire Universe. The only way to stop it seemed to be to kill Charley – but The Doctor.