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 woman who knows the truth of the book, saying it is an evil and cursed book that should not be in Obadiah's hands. 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 Swampfolk 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.

Reward
IF you gave the book 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.