Omega

Omega is one of the greatest heroes of the Time Lord history. The Omega legend says that this brilliant solar engineer sacrificed himself in the detonation of a star, this created the initial power source for time travel. Omega survived however, and was dragged through a black hole into a world of anti-matter where most objects were created by his will. He was lonely, so he projected a light beam through the black hole and began to drain the energy from Gallifrey, which he wanted to rule or destroy. The three Doctors were dispatched by the Time Lords to defeat Omega and they eventually did, and this created a massive explosion. The cause of this explosion, was the second Doctor's recorder, which did not belong in a world of anti-matter. They had discovered that under his protective mask, there was nothing left of Omega, only his will, the anti-matter had consumed his body. They thought that Omega had been destroyed, but however he return with the help of the Time lord Hedin. Omega then almost succeeded in bonding with the fifth Doctor and stealing his body to return to Gallifrey.

Hedin was a misguided Time Lord who attempted to assist Omega in his return to Gallifrey. He saved the Doctor's life at the cost of his own, and Hedin then was unable to see the Doctor killed for his own crime, so he threw himself between the Doctor and the Castellan's gun.

Anti-matter is the opposite of matter. It exists in an anti-matter universe parallel to our own and it is deadly to anything, which is not constructed of anti-matter.

Omega was in The Three Doctors and Arc of Infinity.