Ep 12. Pyramids of Mars (4)

In thwarting Sutekh's bid for freedom, the Doctor gets too close and loses his own, becoming Sutekh's "plaything" for the next century. But Sutekh sees a way of using the Doctor to destroy the power generator on Mars that keeps him paralyzed and entombed on Earth. Then he can finally unleash himself upon the universe, destroying all life, everywhere.