Anti-time

Anti-Time is a dimension inhabited by Neverpeople. Some, if not all, of these people are Time Lords who were thought to have been executed for treason by the Celestial Intervention Agency. The device used for the executions, the Oubliette of Eternity, was supposed to erase the Time Lords from history, but instead transformed them into Neverpeople.

The Neverpeople attempted to destroy Gallifrey by using a casket of Anti-Time. However, the Doctor saved Gallifrey by materialising his TARDIS around the Time Station, thus containing the force of the anti-time explosion. (BFA: Neverland)

Unfortunately, anti-time began to seep into the normal universe as historical anachronisms caused the Web of Time to break down. Both the Doctor and his TARDIS were "infected" by anti-time and/or its inhabitants. (BFA: Zagreus)