When the Doctor was a very young boy, he was informed of a prophecy by the Cloister Wraiths, that a creature forged from two mighty warrior races would stand in the ruins of Gallifrey at the end of time. This prophecy terrified the Doctor into fleeing Gallifrey, and the Time Lords later tracked him down in order to make him confess what he knew.
Identity of the Hyrbid Edit
So, after 12 episodes of characters like Davros and Osgood foreshadowing its existence, what was the hybrid in the end? Well, a lot of theories get thrown about in Hell Bent (including Maisie Williams, or even the Doctor himself via the triggering of TV Movie haters), however the suggestion which takes prominence in the discussion is that the Hybrid is Doctor and Clara being together.
Yup, that's right. It's incredibly likely that Billy Hartnell fled Gallifrey out of fear of himself and Clara boning at the end of existence. Sp00py.
A dumb storyarc that IGN praises as a work of genius for some stupid reason.