Kasaavin

The Kasaavin were beings that came from a realm beyond N-Space.

Biology
Manifesting in N-Space while allied with, individual Kasaavin resembled a white shining humanoid, but explained to the Thirteenth Doctor that this was because the human shape "amuse[d them]". When they emerged through a surface, they temporarily took the image of that which they had passed through. (TV: Spyfall)

Powers and abilities
While in N-Space, the Kasaavin could phase through walls, even those of the Doctor's TARDIS. The Kasaavin were capable of transporting beings to their native realm. (TV: Spyfall)

History
The Kasaavin planned to "take" the Doctor's universe. They, along with Daniel Barton, were unknowingly part of 's plan to destroy humanity and rain destruction across the universe. However, this plan was ultimately foiled by the Doctor and her companions, who forced them back to their own realm. When the Doctor revealed that the Master intended to betray them, the Kasaavin took the Master back with them as revenge for him using them for his own motives. (TV: Spyfall) The Doctor, however, suspected that the Master ultimately escaped the Kasaavin, but was unable to find him. (TV: Fugitive of the Judoon) The Master later appeared from The Boundary after it suddenly connected to Gallifrey, confirming that he did eventually manage to escape the Kasaavin. (TV: Ascension of the Cybermen)

Behind the scenes

 * The Kasaavin were voiced by Struan Rodger.