Arkinen

Arkinan was a ruthless dictator who destroyed planets and civilisations as a hobby. He was stopped by the Second Doctor and Jamie McCrimmon. They swapped his mind with that of a cafeteria worker named Sid. The Doctor checked 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 as he refused the opportunity to cause destruction in the universe again. (ST: The Old Rogue)