OMEGA  

First and foremost of the ancient Gallifreyans' Solar Engineers.  Using his Stellar Manipulator (also known as the Hand of Omega), he detonated a star into a supernova, the tremendous energy of which was required for the Gallifreyans to achieve mastery of time travel and become Time Lords.  However, when the supernova suddenly collapsed into a black hole, Omega was trapped and projected into an anti-matter universe.  He blamed the Time Lords for his predicament, and swore revenge.  When Omega tried to escape, the Time Lords summoned the Three Doctors to fight him.  Omega then discovered that he could not leave the anti-matter universe, because anti-matter had totally destroyed his physical being, and only his mind was left.  He was presumed dead in a matter/anti-matter explosion caused when his anti-matter being came into contact with the Second Doctor's positive matter recorder (RRR).
 However, Omega survived and returned from the anti-matter universe by using the Arc of Infinity, a collapsed Q-star.  He allied himself with Councillor Hedin, invaded the Matrix, and tried to bond physically with the Fifth Doctor, so that he could remain in our universe.  The Doctor then used a matter converter to banish and, presumably, destroy Omega before a huge anti-matter explosion could annihilate Amsterdam (6E).