My idea when I came up with the name is that the main entrance of the dungeon only opens during a new moon (but there are plenty of other alternate entrances that were created as the new denizens of the dungeon expanded it) but I wouldn't be opposed to adding in additional interpretations. I also lean towards the last option in your list, there probably are rferences to it and maybe it was worshipped by the original builders of the dungeon, but maybe no one currently living there knows exactly what it was.