Kasaavin realm

The Kasaavin realm was a dimension from where the Kasaavin came, and which the Thirteenth Doctor believed was external to her universe.

Structure
It connected to multiple time periods on Earth. (TV: Spyfall) The Doctor found that she could not enter it using the TARDIS. (TV: Fugitive of the Judoon)

History
Having been exposed by the Doctor as planning to betray the Kasaavin, was forcibly taken with them to their realm. (TV: Spyfall) Considering the possibility that the Master was able to escape the Kasaavin realm, the Doctor unsuccesfully searched the universe for him. (TV: Fugitive of the Judoon)