Enjoyed the episode, bravo Capaldi. Great job all around on the production side of things.
I think the Doctor is referring to himself as 'The Hybrid' of prophecy. When the Doctor confesses that he knows what the hybrid is, he says the Time Lords knew it was coming, 'like a storm on the wind'. And there's all those repetitive scenes of "I am in 12".
I expect to be surprised by the nature of 'The Hybrid' (especially if I'm wrong about it being the Doctor, lol); since The Doctor has returned to Gallifrey, throwing out an obligatory "Valeyard" guess.