Borusa

Borusa was a Time Lord. He was a teacher of the Doctor and eventually President of the Time Lords. However he wanted immortality.

History
Using the forbidden time scoop Borusa kidnapped the first 5 incarnations of the Doctor (save the 4th 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 5th 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 5th 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 3 Doctors the 5th 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.