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, which was stolen from him by some Swampfolk.

Walkthrough
Obadiah tells you that the book he wants back is to the East, at the Ritual Site. Upon leaving Obadiah's home, the player is met by Marcella, a missionary spreading faith across the wastes, after listening to her ramble, you find out that Obidiah actually wants the book for more than just keepsakes, and plans to use it for a ritual to control the Swamp Folk, and under no circumstances should it be returned to him. She requests that, upon finding the book, you find her at the Disaster Relief Outpost, in western half of Point Lookout.

But, to make either decision, you have to first find the book. Head to the Ritual Site and look out for the Swamp folk guarding the entrance to the site. Look for a small door in the ground and head in. Now find your way to the bottom and you will see a large table with a dead body with a Ritual Knife stuck into the head. In front of it there is a pedestal with the book, take it. Then head back to either Marcella or Obadiah.

If you take it to Marcella, you will find her dead in her tent, with her last words in the table in the form of a tape, in them she begs you to destroy the book. You have to head into the deepest chamber in Dunwich Building back in the capital wasteland to destroy it. After finding the mysterious statue of the Dunwich Building you can activate the statue and the book will ignite in flames. Reactivating the statue it will state that it makes you feel uncomfortable for standing next to it. You will be awarded with good Karma.

Or, you can take it for Obadiah for a cap reward, and if you follow him, he'll go to his basement where he prays with the book in front of a mutilated swamp folk 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 changes, so the nonsense about him gaining control of swamp folk seems to be false. Though you do get a new chat option, allowing you to mock him about having bodies in his basement!

Reward
If you gave the book to Obadiah Blackhall, you'll get his reward of 1000 caps and 100xp for completing the quest. Note: You do not receive any negative Karma for returning it to Blackhall.

Alternatively, destroying the book with the Obelisk in the Dunwich Building gives you a positive karma boost and the 100xp. Though it should be noted giving it to Obadiah has no penalties to your Karma.

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.