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 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 amongst the charred remains of a building and head in. Now find your way through the chambers to the bottom of the eerie 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, 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, or you could sneak out, if you have a good sneak skill. 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 as "Die thief" is written in blood on the side of the tent, hinting that she was believed to have the book. Killing Obadiah after talking to Marcella doesn't affect this outcome because the hired smugglers will still kill her. 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, entering the tunnels beneath. After finding the mysterious obelisk in the Dunwich Building you can activate the creepy obelisk and the book will ignite in flames. Reactivating the obelisk will state that it makes you feel uncomfortable standing next to it. You will be awarded with good Karma.

Or, you can take it to 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. 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 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. (confirmed all platforms)


 * It's possible to miss Marcella's encounter if you exit through the kitchen and thus will not show up at the tents. You will only be able to end the quest by giving the book to Obadiah if you miss the encounter at the front door. (unconfirmed)


 * 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. (PC, 360, and PS3 confirmed) 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 (on PC!) use the console to manually update your questlog by using the command 'setstage xx002f47 240', whereas 'xx' must be replaced through the number the Point Lookout-DLC gets in your load order. Your questlog now lets you choose between helping Obadiah or Marcella.


 * 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. (confirmed PC, PS3, Xbox 360)


 * For some people, listening to Marcella's holotape, causes the game to crash. Which makes it impossible to complete the quest in the method of "giving her the book instead of Blackhall", without using the console command, "setstage". (confirmed PC)