Zagreus

Zagreus was a character in a Gallifreyan nursery rhyme and a boogeyman to the Time Lords.

History
Legends stated that Rassilon himself destroyed Zagreus when the Yssgaroth came into the Universe in the events around the creation of the Eye of Harmony. It was said by the Neverpeople that when a casket of Anti-Time was detonated at the Capitol, chaos would engulf the Universe and the empire of Zagreus would form. When the Eighth Doctor prevented this event, he absorbed the Anti-Time particles into his own body,. He became a dark being and the embodiment of Zagreus (BFA: Neverland), although his true personality remained intact if submerged by Zagreus's strength. The TARDIS was fractured into a similar state, torn between its attempt to help the Doctor and its resentment at his willingness to sacrifice it for Charley. While the Doctor tried to regain control of himself, the Zagreus-TARDIS sided with Rassilon to turn the Doctor fully into Zagreus, to serve as Rassilon's assassin in exchange for Rassilon freeing the TARDIS from the Doctor's 'enslavement', the two of them apparently destroying the TARDIS's shell to sufficiently traumatise the Doctor to allow Zagreus to take control. However, the TARDIS' 'good' side retained enough control to generate various holograms based on the Doctor's previous three selves, the holograms helping the Doctor remember who he was.

Despite Rassilon's efforts to reaffirm the Doctor's status as Zagreus, both personalities rebelled against Rassilon's plans, the Doctor refusing to kill while Zagreus refused to be anyone's puppet. With the Zagreus-TARDIS having been destroyed by Charley, Leela and Romana, the Doctor-Zagreus banished Rassilon into the anti-time universe before the good TARDIS was able to provide his companions with a cure to the anti-time energies in the form of pure zero matter, simultaneously restoring the TARDIS's physical form to normal. However, although the zero matter stabilised the Zagreus 'infection' within the Doctor, the Doctor had to remain in the universe of anti-time to prevent the anti-time still within him from destroying the universe. (BFA: Zagreus)

The anti-time energies were purged from the Doctor by Rassilon when he entered the anti-time universe. This let Zagreus manifest as an independent spirit. In this form, Zagreus could possess the bodies of the dead, and confronted the Doctor in the body of Perfection, the 'perfect woman'. She tried to trick the Doctor into taking her into the main universe, but the Doctor saw through her deception when she mentioned various details that only someone from his universe could know, leaving Zagreus trapped in anti-time, while he and Charley returned home. (BFA: The Next Life)

Prubert Gastridge once voiced Zagreus in a cartoon. (EDA: The Tomorrow Windows)

Zagreus Rhyme
Zagreus sits inside your head,

Zagreus lives among the dead,

Zagreus sees you in your bed,

And eats you when you're sleeping.

Zagreus at the end of days,

Zagreus lies all other ways,

Zagreus comes when time's a maze,

And all of history is weeping

Zagreus taking time apart,

Zagreus fears the hero heart,

Zagreus seeks the final part;

The reward that he is reaping.

Zagreus sings when all is lost,

Zagreus takes all those he's crossed,

Zagreus wins and all is cost,

The hero's hearts he's keeping.

Zagreus seeks the hero's ship,

Zagreus needs the web to rip,

Zagreus sups time at a drip,

And life aside, he's sweeping.

Zagreus waits at the end of the world,

For Zagreus is the end of the world.

His time is the end of time,

And his moment time's undoing.