Blood Moon Sanctuary?

[syrbeck]

So I find this dungeon in my game, located on the NW corner of the map, close to the ocean. There appears to be a quest to get inside the dungeon, but I can't figure out how. I have no idea which mod added this dungeon. I've gone through all of the likely mods with no mention of Blood Moon Sanctuary. Does anybody know what mod adds this dungeon, or better yet, how to solve the riddle to get into it?