Divergence

The Divergence were a race that would have evolved after the Time Lords. Rassilon didn't want to commit genocide, so he locked them away in a time loop to be trapped until the end of time.

In Cardington, 1951, Doctor Stone and Reverend Townsend opened a doorway to the Divergence, this failed and killed Stone and Townsend.

At the end of time, approximately sixty billion years in the future, they nearly did get out. Uncle Winky's Wonderland was built over Rassilon's lab, but Uncle Winky didn't open the door when he should have. (BFA: Zagreus)