Scibus

Scibus, along with Tarpok, was a Silurian companion of Icthar, the sole survivor of the Silurian Triad.

In 2084, they tried to take over Sea Base 4 and start a nuclear war to wipe out humanity. They were defeated by the Fifth Doctor. Scibus died from exposure to hexachromite gas just after beginning the missile launch sequence. (TV: Warriors of the Deep)