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The Kasaavin were beings that could phase through walls, even those of the Doctor's TARDIS. They took humanoid and shining bright white light form while allied with the Spy Master, but explained to the Thirteenth Doctor that this was because the human shape "amuse[d them]." When they emerged through a surface, they took the image of that which they had passed through.
The Kasaavin planned to "take" the Doctor's universe. (TV: Spyfall)