Omega

Backstory
Omega is one of the most significant figures in Time Lord history, sometimes called the first Time Lord. In the "Dark Times", it was Omega ("The Three Doctors"), in collaboration with Rassilon, who pioneered the science of "stellar manipulation", creating the Hand of Omega ("Remembrance of the Daleks"), a device which could "customise stars", the energy from which, harnessed would enable the Time Lords to have time travel. (This may relate to the Eye of Harmony ("The Deadly Assassin"), a black hole captured and fixed beneath the Panopticon and said to power all of Time Lord civilization, including TARDISes.)

In official history, both Omega and the Hand were lost in the experiment. But the Hand of Omega did survive (and was eventually hidden by the Doctor on Earth as part of an elaborate trap for the Daleks), as did Omega, though without access to the Hand. Omega (but not the Hand) was transported through the black hole to a universe of anti-matter, which he was able to shape through by exerting his will over the power of the black hole's singularity. There he stayed, trapped for millennia, still wearing the heavy gauntlets, armour and helmet he had designed to protect him from the corrosive effect of the anti-matter. In the beginning, he shaped his new world into a paradise, but as the centuries rolled by he grew weary and depressed, feeling abandoned by his fellow Time Lords, and the landscape slowly transformed into a drab, grey desert.

The First Revenge
Eventually, Omega hit upon a plan of revenge. The universe that had become his home was unstable, unable to exist without the will of a powerful mind to give it form; he was trapped. But if he could capture a Time Lord, he could force them to take his place, allowing him to leave and wreak vengeance on Gallifrey. He drained power from the Eye of Harmony, preventing Gallifrey from interfering, and sent simple servants through the black hole to Earth, where he had detected a suitable Time Lord: the Third Doctor. Omega's amorphous servitors brought the Second and Third Doctors to his domain, converted into anti-matter, and Omega forced them to agree to his plan. However, when he removed his helmet to prepare his departure, he discovered that the anti-matter universe had completely dissolved his physical body: he existed only as a mind, and could not leave his universe. Consumed by rage and despair, Omega swore to destroy all things, and the Doctors resolved to put an end to his madness. They discovered a piece of matter,the Second Doctor's flute, which had been protected from anti-matter conversion on its journey though the black hole. They presented it to Omega, who, believing it to be his only salvation, removed it from it's protective force field, supposedly obliterating himself and his entire realm. ("The Three Doctors")

The Second Revenge
Omega was thought destroyed, but certain elements within Time Lord society sympathised with his plight and held out hope that he still lived. One such Time Lord was Chancellor Hedin of the High Council, whose faith was rewarded when he made contact with Omega, who had somehow survived, still trapped in the universe of anti-matter. Omega had devised a new plan to escape hs prison; somehow he had gained control of the dimensional gateway known as the Arc of Infinity, through which he could enter the universe of matter. In order to have a physical form, however, Omega needed to bond with another Time Lord, impossible without precise biological data about his subject. Hedin transmitted the Doctor's bio-data extract to Omega for this purpose, seeing a chance for his hero to have revenge on the Time Lord who had defeated him.

Omega landed his TARDIS, shielded by the Arc, in Amsterdam on Earth, securing the area with the aid of his new servant, the Ergon. This would be the location of his physical manifestation. He steered the Fifth Doctor's TARDIS into the Arc and established an initial link with him, but this was detected by the High Council, who (unaware of Omega's true identity) brought the Doctor to Gallifrey in order to kill him, severing the link. This was all part of Omega and Hedin's plan: they rigged the execution so that it would hide the Doctor and Omega in the Matrix, safe from Time Lord detection.

Omega blackmailed the Doctor to force him not to interfere with the transfer by holding his companion Tegan hostage, but this gave the Doctor vital clues as to Omega's physical location. Omega shifted the Arc to Gallifrey in order to gain control of the Matrix, using its power through his link to the Doctor to fuel his transfer through the Arc, but the Doctor tracked him down and sabotaged his equipment in Amsterdam, forcing Omega to step into the physical universe before the transfer can be made stable. His new body was a replica of the Fifth Doctor's, but quickly begins to decay back into anti-matter. Again thwarted and mad with defeat, Omega willed the acceleration of his conversion to anti-matter in order to destroy the Earth rather than return to the universe of anti-matter, but was destroyed by the Doctor with the Ergon's matter conversion gun. ("Arc of Infinity")