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)

Victims of the Kasaavin's attacks were transported to the realm to have their DNA rewritten, killing them in the process. Despite this however, the Kasaavin were "not as stable in [N-Space] as they'd like to be", and were thus unable to kill Yasmin Khan and the Doctor because they were "fizzing with artron energy". Additionally, in the case of the Doctor, her DNA didn't match the "rest of humanity". As a result, the realm was confused and released them unscathed.

Curiously, the realm did not affect Ada Lovelace either. This was likely due in part to her role in the Kasaavin's plans as someone close to Charles Babbage, owner of the Silver Lady that allowed the Kasaavin stability within N-Space. (TV: Spyfall)

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 from the Kasaavin realm, the Doctor unsuccessfully searched the universe for him. (TV: Fugitive of the Judoon) As later revealed, the Master did in fact manage to escape somehow and return to the universe. (TV: Ascension of the Cybermen)