Maybe it was a lost alien race that worships darkenss and consumed the moon! Either way were gonna adventure deep into the moon and send all those baddies back to hell!

Given the presence of Fallen on the Moon in the [i]Law of the Jungle[/i] trailer and their presence on Earth, shown in the E3 trailers; I would say that the Fallen are maybe the most aggressively entrenched enemy we will encounter. -I think this because it seems to me that each enemy has a planet to themselves (at least for the most part) which they have claimed and dominated: (Venus- Vex, Mars- Cabal, Jupiter- 5th*, Saturn- Hive*) because we have only seen one form of enemy on each planet. -That being said, we are left with only three more geomantic symbols to account for; Mercury, Sun, and Moon. Mercury I can see being the point of conflict for all enemies; the potential for energy harvest on Mercury would probably drive everything to fight over it. The Sun, I feel, is the unifying symbol and represents the Solar system instead of just Sol. -I believe the Moon belongs to the Fallen, perhaps the Hellmouth serves as (one of their) their base(s) of operations. There's some heavy reference to the darker side of mythology going on with the Fallen. It seems fitting the Hellmouth would be the name of something they consider cozy. Plus I can imagine them swarming out of it like locusts, descending upon earth like a lunar-borne scourge. *-My personal guesses to be elaborated on as digression and this post is getting long.

Sentinel Prime.