the Point Lookout DLC has a very Lovecraftian quest. There's an old manor with an old man in a wheelchair who doesn't show up on your compass at all. He wants you to get an ancient tome for him from another location. In the end, if you give it to him, he suddenly can get up from hiw wheelchair, walk into the basemant, and does some strange ritualistic things down there.
The quest's name is "The Dark Heart of Blackhall", and here is a link to it's page on the Fallout wiki: http://fallout.wikia.com/wiki/The_Dark_Heart_of_Blackhall