If I'm right, as soon as you go down to the basement the thing will be waiting for you right outside the elevator/platform.
And I don't remember if the Library needed a key at all, I mean, I killed both the basement one and the necromancy chamber one, and yet, I don't think the keys I've got from them were of any use


Try mindreading the Goblin in the library.

IDK what to put here, so don't bother reading it, you're just wasting your time... STOP READING IT!