Zagreus

''Zagreus sits inside your head.

Zagreus lives among the dead…

Zagreus sees you in your bed

And eats you when you're sleeping''


 * See also Zagreus (audio story)

Zagreus was a character within a Gallifreyan nursery rhyme and was a boogeyman to the Time Lords (Similar to how the Toclafane were like the bogeyman stories The Doctor and The Master used to hear when they were children).

Legends stated that Rassilon himself destroyed Zagreus when the Yssgaroth came into the Universe as the Eye of Harmony was created. It was said by the Neverpeople that when a casket of Anti-Time was detonated at the Capitol then chaos would engulf the Universe and the empire of Zagreus would form. When the Doctor prevented this attempt, he absorbed the Anti-Time particles into his own body becoming a dark being and the embodiment of Zagreus. (BFA: Neverland)