Divergence

The Divergence were a race that would have risen to power 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. "Divergence" was a name given to the creatures by Rassilon. It is unclear what their actual name was, if they had any at all.

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)

The Divergence may or may not be related to the Enemy from EDA: Alien Bodies or the Yssgaroth of FP: The Book of the War.