Borusa

"I am Lord President of Gallifrey! And YOU are the notorious renegades! We shall see who is believed."

- Borusa to the four Doctors.

Borusa was a Time Lord and the main antagonist of the Doctor Who special episode "The Five Doctors", who had already appeared in three previous episodes. He was a teacher of the Doctor as a child and eventually President of the Time Lords. However, he wanted immortality, which made him become evil.

Origin
When the First Doctor was young, Borusa was his teacher at the Time Lord Academy. At that time he was a member of the "Prydonian Chapter".

Borusa first appeared as Lord Cardinal of the High Council of Time Lords, when the Fourth Doctor was sent to his home planet Gallifrey, where the Master tried to extend his life, as he was at the end of his thirteenth and therfore final incarnation. At all his three further appearances, Borusa would encounter the Doctor in an entirely different incarnation.

Borusas Evil Plan
Using the forbidden time scoop Borusa kidnapped the first five incarnations of the Doctor (save the Fourth who was trapped in a time eddy when Borusa tried to kidnap him) and sent them into the Death Zone to remove the defences of the Dark Tower. It was said the first Time Lord President Rassilon had discovered immortality and still lived in the Dark Tower, thus Borusa wanted to access the Tower. He sent the Master into the Death Zone to rescue the Doctor so as to dissuade suspicion. The Fifth Doctor escaped the Death Zone using a transmat device he found on the Master. Borusa then framed the Castellan for using the time scoop, leading to the Castellan's death.

When the Fifth Doctor found the hidden room where Borusa was using the time scoop, Borusa used the coronet of Rassilon to place him under mental control. The other Doctors had entered the Dark Tower and deactivated the force field, enabling Borusa to transmat into the Tower. However with the help of the other three Doctors the Fifth Doctor was able to break free of mental control. Over the sleeping body of Rassilon an image of Rassilon appeared and told Borusa to take the Ring, which would give him immortality. When Borusa put on the ring Rassilon revealed others had entered the Tower to gain immortality and Borusa was turned into one of the statues in the Tomb, gaining immortality.