Kasaavin
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The story in question has not yet been released in its entirety. Please exercise caution in asserting any more than is actually present in the narrative so far. Be aware that certain facts which appear to be true may not be true once the story is completely published.
The Kasaavin were beings that could phase through walls, even those of the Doctor's TARDIS. They took humanoid form while allied with the Spy Master, but explained to the Thirteenth Doctor that that was because this 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)
