Mestor

Mestor was the leader of the Gastropods which had conquered Jaconda. In 2310, 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.

After his plan was thwarted, Mestor tried to take over Azmael's mind and body, he then tried to force him to mind-link with the Doctor, however due to Azmael forcing a thirteenth regeneration and the Doctor destroying his psychical body, he was exorcised and died. (TV: The Twin Dilemma)