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The Tomekeeper is a ghost found in the Realm of Shadow (accessed from The Terminus). He is a relatively rare ghost, taking possibly dozens of tries to find. He is usually just inside the dungeon, or in the next room, and has been verified to be in Normal, Hell difficulty and Inferno (likely any difficulty.)
If you want to farm him for the Haunted achievement, start at the quest The Black Soulstone: Soulstone Chamber (Act II), and enter The Terminus waypoint. The Tomekeeper can be found ANYWHERE within the "Realm of Shadows"