The Doctor believed he left Rose behind with a part-human version of himself, to live happily ever after. So what will he do when he discovers that the Metacrisis Doctor is not the Doctor at all, but rather the Valeyard- the evil fulfillment of an old, dark prophecy? Ten/Rose
In which the 13th Doctor's descent into becoming the Valeyard is witnessed first hand by Isobel. Unable to leave, she takes desperate measures and unwittingly makes things worse. Dark 13, Valeyard 13 is canon change my mind.