I was thinking it might be fun to have some light collaboration on what the titular “Dark Moon” actually is. Some ideas I’ve come up with are:

• celestial event of some kind, an eclipse or alignment of stars
• a magical cycle, the Dark Moon is a Thing that happens every x years, has an effect on the world but not necessarily astrological
• an artifact of immense power
• some sort of secret society
• the name of a wizard
• my favorite: Nobody actually knows in universe either. There’s texts referring to the Dark Moon but nobody can agree on what that actually Means

Would love to see what sort of ideas everyone else comes up with!

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. 

Perhaps the Dark Moon is tied, somehow, to perception? If you believe it to be a diety, it is. If you believe it to be an event, so it is instead. If you think it's just a story, well honestly it's kind of insulted by that, so good going, better not change your mind.

I agree that there should be multiple theories in world since there's multiple authors for the dungeon. In my (first..?) entry, there's an occult order that believes the dungeon's location is mystically connected to the moon, but I left that pretty vagueand didn't confirm or deny if they were right.