Arkinen

Arkinan was a ruthless dictator who destroyed planets and civilisations as a hobby. He was stopped by the Second Doctor and Jamie McCrimmon who swopped his mind with a cafeteria worker named Sid. The Doctor then proceded to check up on him every ten years, once in his third incarnation and ten years after that, in his fourth incarnation. After the Fourth Doctor's visit, Arkinian was finally rehabilitated from his crimes, as he refused the opportunity to cause destruction in the universe again. (ST: The Old Rogue)