Sancroff was the First Knight of Velyshaa. After the Knights were defeated in 3562, he was sentenced to permanent banishment to a security planet, under the care of Ruthley. Having had many years to think about what he had done, he came to realise that while at the time it seemed like a great adventure, his career as a Knight caused much death and destruction, and he repented of his crimes. He thus willingly allowed bio-engineered assassins sent from Calfadoria to kill him.
In an alternate timeline, the Seventh Doctor rescued Sancroff from the assassins. This allowed the Knights to locate and rescue him, and he once again led them in conquest. (AUDIO: The Sirens of Time)