Mortakk's crimes included conducting genetic experiments on Silurians and Sea Devils. He was tried and believed to have been executed before the great hibernation. However, his base was located in an abandoned nuclear research centre at Darkmoor and the Seventh Doctor, along with Bernice Summerfield and the Silurian scientist Thactus of URIC, travelled in the TARDIS to confront him.
Mortakk had indeed survived. He created a gas with mutagenic properties that forced Silurians and humans to evolve into hybrid creatures with both reptilian and mammalian qualities. This brought him into conflict with his enemies. Mortakk fought the Doctor in a sealed room where the gas was released. Due to his Gallifreyan biology, the Doctor was not affected and Mortakk was killed in the battle. (COMIC: Final Genesis)