I got the key from Kreug when I killed him. I didn't accept his offer to join... hehe!
But I missed that he dropped a key for a while. It was very hard to see in the particular spot it landed on the floor. I went back and looked at a later time, and there it was!
BLKCat With Poor Eyesight
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...Events are cowards: they don't occur singly, but instead they run in packs and leap out at you all at once.
(Excerpt from "Neverwhere", by Neil Gaiman)