Mestor

Mestor was the leader of the Gastropods which had conquered Jaconda. He enslaved Azmael with his mental abilities and forced him to abduct Romulus and Remus Sylvest. He planned to use the twins to solve the equations needed to explode Jaconda's sun and spread Gastropod eggs throughout the universe. The plan was defeated and Mestor was killed by the Sixth Doctor while attempting to take control of his mind and body. (TV: The Twin Dilemma)

Behind the scenes
Mestor was immortalized as a genus of prehistoric gastropods, Mestoronema.