If you are familiar with this DLC, you know there is a mission in which you are sent to retrieve a very special book for an old man living in one of the mansions on Point Lookout. The book was called the Krivbeknih and, later on, you find out that it supposedly has some kind of dark power and needs to be destroyed. You can either give it back to the man or destroy it by taking the book to the Dunwich building in the Capital Wasteland.
As my brother is interested in occult stuff, he found this quest really interesting. Before even doing anything more with the quest, he went on Fallout Wiki and looked up all the info he could find on the book and what would happen after. I thought it was stupid of him to get everything spoiled, if he now found it such an interesting quest, but he said he somehow felt some kind of greater importance to it.
After shooting through the Ritual Site, killing all of the swamp-folk there, he decided to destroy the book, which forced him to go back to the Capital Wasteland (the original game world). At this point, he didn't have enough caps to buy another river-boat ticket, so he killed Panada (a trader in Point Lookout) at the docks, and took all of her caps. I found this very strange, as my brother isn't playing as a bad character and would always do the moral and right thing, wanting good karma... but not this time.
After coming back to the riverboat docks in the Capital Wasteland, he began making his way towards the Dunwich building. Girdershade, the place where you get the Nuka-Cola Challenge, was the closest location discovered, so he went there, only to realize that the place was deserted. The two people that used to live in the little town had vanished. We thought it must have been a game bug, as my brother never killed them. We ignored it and he moved on.
Finally, he arrived. He went through the levels pretty quick, witnessed the hallucinations and the self-opening doors. We weren't really scared by any of that because we knew it was going to be there. He got to the under-chambers and placed the book on the creepy-looking altar, and it somehow completed the mission by destroying the book.
At this point, my brother figured it would be a good time to head to Rivet City to sell some stuff he had gotten from Dunwich. Guess what? Not a single person could be found there. At this point we got really freaked out. We checked the other settlements: Megaton, The Citadel, Tenpenny Tower, and all of them were empty. We restarted the game and reloaded, but nothing helped.
We figured we could go back to Point Lookout and see if there was anyone left there. The only problem was that Tobar (the guy who owns the boat) wasn't around anymore either, so we couldn't buy a ticket. When my brother was about to leave the boat again, he was attacked; attacked by the very same people missing from all of the settlements.
It must have been over a hundred people just charging the boat—throwing grenades, shooting missiles, wanting my brother dead. He didn't really have anything that could kill all the people at once, so we thought it would be a good idea if he went into the little cabin on the boat and wait for them to come. That didn't work out as we planned.
They didn't run in; instead, someone with a fat man launched a mini-nuke into the cabin, completely blowing my brother into pieces. Right after that, the game froze and we had to restart it.
Now, everything was fine. No one attacked us and everyone was exactly where they were supposed to be.