The Dark Heart of Blackhall

 is a Fallout 3 quest that is added to the game with the Point Lookout add-on. The player is tasked with finding Obadiah Blackhall's book, the Krivbeknih, which was stolen from him by some Swampfolk.

Walkthrough
You were a dumb prick who rewrote everything on here to seem smert! :D

Or, you can take it for Obadiah for a cap reward. If you follow him, he'll go to his basement where he prays with the book in front of a mutilated Swampfolk corpse. Giving him the book does not seem to do much of anything, other than him going to the basement to pray in front of the corpse. Nothing else changes, so the nonsense about him gaining control of the Swampfolk seems to be false.

Reward
If you give The Krivbeknih to Obadiah Blackhall, you'll get his reward of 1000 caps and 100xp for completing the quest.

Alternatively, destroying the book with the Obelisk in the Dunwich Building gives you a positive karma boost and the 100xp.

Also, you can give the book to Obadiah, get the caps, then kill him for no karma loss. Now you can take the book to Dunwich and destroy it, but you'll get no reward save for peace of mind.

Bugs

 * It is possible that upon the return to the Capital Wasteland to finish the quest, the map marker will be located in the center of the map instead of at the Dunwich Building, which is located in the bottom left corner of the map Or in the center of the map next to Springville elem. (PC and 360 confirmed)