Cultist's Diary is a book found in Death of the Outsider.
It has begun. When I woke this morning my shoulder felt stiff and insensate, and when I inspected it there was a thread of pure, silver stone running across my flesh. At last! I am merging with the Void. Oh, to be removed from this reality and exist eternal in the next! To see the subtle shifts and patters of the Void, to be closed off from all contact with lesser creatures! I am eager to see its progress.
The stone spreads. I feel it humming with heat as it enclosed my very heart. Sometimes I am breathless with it. I can barely listen to the others... At times, my tongue won't move for hours, and I am mute.
My left eye has been entirely subsumed by the Void. I can see strange shifts in the light, but it is mostly endless darkness. I find it difficult to focus on my work.
My pen slips from my hand even as I try to write these words. My fingers have grown cold and numbed to all sensation. Some have even fused together in an amalgam of stone.
I am exalted.
This book can be found inside the Shindaerey North Quarry on a bed, during the mission A Hole in the World.