Professor Donald Eustace Stark was a scientist.
Stark worked with Grace Holloway on a project to merge Time Lord and human DNA. However, the sample Holloway had obtained was from the Deathworm Morphant that the Old Master had used to survive his execution by the Daleks. Stark used the sample on himself, becoming a human-morphant hybrid.
The morphant's hunger made him eat people and absorb their knowledge, growing larger. But since Stark did not have the Master's strength of will, he was overwhelmed by all the memories of his victims. The Eighth Doctor was able to destroy him with a helicopter full of explosives. (COMIC: The Fallen)