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
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 speech, 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 Swampfolk, 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.

However, to make either decision, you have to first find the Krivbeknih. Head to the Ritual Site and look out for the Swampfolk guarding the entrance to the site. Look for a small door in the ground and head in. Now find your way through the chambers to the bottom of the eerie ritual area. Here you will see a large table with a dead Swampfolk body with a Ritual Knife thrust into the body. Directly behind the table is pedestal with the book inside, floating in "freshly spilt blood." You have the choice to leave it alone, or remove it from the chamber. It is advised that you arm yourself before claiming the book, and possibly even place mines near the entrance to the ritual section of the chamber. Upon taking the book, Swampfolk reinforcements will begin to pour into the area, furious with your actions. From here you can return the book to Obadiah or bring it to Marcella.

If you take it to Marcella at the Disaster Relief Outpost 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 Krivbeknih. If you wish to destroy the book, you have to venture to the deepest chamber in the Dunwich Building back in the Capital Wasteland. After finding the mysterious statue in the Dunwich Building you can activate the creepy statue and the book will ignite in flames. Reactivating the statue will state that it makes you feel uncomfortable standing next to it. You will be awarded with good Karma.

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.  (PC Confirmed)