Chronopolis

Chronopolis was a city in an alternate timeline created by the Valeyard where that dark incarnation won in the trial against the Doctor. Following his claim of the Doctor's past regenerations and thus achieving true immortality, the Valeyard crafted a city within the Time Vortex around the TARDIS which protected it from harm. Called Chronopolis, it was within its many chambers that the Valeyard placed a sample of every species that had destroyed. The nature of this construct meant that it projected a different appearance to whoever saw it. (BFA: He Jests at Scars...)