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
Enter Blackhall Manor where 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 her faith across the wastes. She claims that Obadiah 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 give it to her instead, at the Disaster Relief Outpost, in western half of Point Lookout.

However, to make either decision, you first have to 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 amongst the charred remains of a building and head in. Find your way through the chambers to the bottom of the ritual area. Here you will see a mutilated body with a Ritual Knife thrust into it, laying on top of a large table. Directly behind the table is a pedestal with the book inside. You have the choice to leave it alone, or remove it from 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 that she was killed in her tent. However, she left a holotape for you; the Swampfolk or Obadiah supposedly hired smugglers to kill her ("Die thief" is written in blood on the side of the tent, hinting that she was believed to have the book). Listening to the holotape, she begs you to destroy the Krivbeknih. If you wish to destroy the book, you have to venture to the Virulent Underchambers in the Dunwich Building back in the Capital Wasteland. You need to access the part where there are metallic corridors near the end of the Virulent Underchambers and then ignore the exit stairs. Enter the lowest level into the tunnels beneath. You will find Jaime worshipping the obelisk. After finding the obelisk in the Dunwich Building you can activate it and the book will ignite in flames. You will be awarded with good Karma.

If you take the book to Obadiah, he'll reward you with 1000 caps. Nothing else changes - he will still go to the basement and pray, whether or not you destroyed the book.

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 karma nor the xp reward.

Bugs

 * It appears that if you take the book from the Ritual Site and head to Blackhall Manor to return it, you will engage in a conversation with Obadiah Blackhall. If you tell him you haven't decided what you're going to do with the book, he will mention Marcella. You then will have the option to say she is dead, even if you have not visited her yet and seen her corpse.
 * It is possible that upon returning to the Capital Wasteland to finish the quest, the map marker will be located in the center of the map next to Springvale Elementary School instead of at the Dunwich Building, which is located in the bottom left corner of the map.
 * It is possible that Marcella won't show up dead in her tent, though the smugglers have conquered her camp. You can however use the console to manually update your questlog by using the command on the PC, where 'xx' must be replaced by the number the Point Lookout-DLC gets in your load order. Your questlog now lets you choose between helping Obadiah or Marcella.
 * Sometimes Marcella's dead body appears just outside Blackhall Manor after you have obtained the book. However the holotape does not appear in the tent.
 * Upon discovering the Krivbeknih in the underground shrine, the player will see that it is resting within a font. From most angles the font looks empty with the exception of a skull and the book itself. However, if the player is to view the font from certain angles it is actually filled with what is presumably blood, which disappears and reappears depending on the player's point of view.
 * For some people, listening to Marcella's holotape causes the game to crash. This makes it impossible to complete the quest in the method of "giving her the book instead of Blackhall" without using the console command "setstage".
 * It is possible for the Ritual Knife to fall through the top of the table and become trapped in the "box" of the table; however, the Ritual Knife may still be retrieved via the rocket launcher or by using other explosives to blow it up out of the top of the "box". It is suggested that a save be made right before entering the burned-out basement for the first time. If you enter the basement and at a later time begin the quest, the items will have spawned when the player first entered the basement and you won't be able to confirm the status of the knife until after you have accepted the quest. You shouldn't enter the basement before you start the quest.
 * It is possible that Marcella will not appear upon exiting the manor.