appearance Warriors of the Deep
click on images to enlarge
Icthar was one of a Triad of Silurian leaders.
Following his revival into a world dominated by humans, Icthar decided, with his fellows, to take over Sea Base 4 with Sea Devil troops and a Myrka, and trigger a nuclear war between two human factions. His familiarity with the Fifth Doctor suggested an earlier encounter. Turlough shot him with a Sea Devil weapon while he was recovering from hexachromite gas poisoning. Though wounded, he attacked and killed Commander Vorshak, but was shot and killed by Turlough. (Warriors of the Deep)
BEHIND THE SCENES
The Target novelisation, Doctor Who and the Cave-Monsters, gives alternative names of all three and does not list Ichtar as one of the three.
Additionally, Icthar appears in the novel The Scales of Injustice, which also establishes that Silurians belong to different races or sub-species, with Icthar belonging to a different sub-species than the one seen in Doctor Who and the Silurians.