Mestor was the leader of the Gastropods that had conquered Jaconda. In 2310 he enslaved Azmael with his mental abilities and forced him to abduct Romulus and Remus Sylvest, planning 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, but, due to Azmael forcing a thirteenth regeneration and the Doctor destroying his physical body with Mosten acid, he was exorcised and died. (TV: The Twin Dilemma)