So I was burning through stacks of lockpicks in the counting house (as you do)
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and I broke into Gortash's Vault, I think it's Vault no 6, and he's got a book there.

Apparently when they were raiding the demon vault, they found the Crown alongside a book outlining the whole scheme to use the crown on an elder brain and create a tamed cult of pseudo-mind flayers. Gortash didn't even come up with it, but the next words in his diary are "This was a scheme so brilliant only I, the Chosen of Bane, could have come up with it!" and I find that hilarious.