- You may be looking for the Master.
In this universe, Koschei had not become the Master. He and Ailla became stranded on Earth when Koschei's TARDIS was irreparably damaged in a battle with a disembodied entity which used robotic bear-like creatures as its foot soldiers and also tried to shroud the world in a web-like substance. His TARDIS was ruined and its control console — the only part that could be salvaged — was cannibalised by agents of the Republic of Great Britain, allowing them a means of travelling between parallel universes. The Republican Security Forces killed Ailla and British Intelligence enslaved the badly injured Koschei, imprisoning him and wiring him to their computers to forcibly extract information.
He was tortured to death several times during his interrogation, using up his remaining regenerations in the process. After meeting himself from a parallel universe, he begged for death. His counterpart duly complied by disconnecting his life-support systems. (PROSE: The Face of the Enemy)