Scibus

Scibus, along with Tarpok, was a Silurian companion of Icthar. According to one account, Icthar was the "sole survivor" of the Silurian Triad. (TV: Warriors of the Deep) According to another account, Scibus was himself one of the Triad, and Icthar's "Second". (PROSE: The Scales of Injustice)

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)