So the Fallen are a bunch of interplanetary scavengers that came looking to loot Earth and the rest of the solar system after The Collapse. They can be from a noble house, but don't necessarily have to be. The current houses I've found are Devils (red), exile (green), Winter (blue) and Kings (gold). There's some more lore behind them in their grimoire cards. Each house is led by a Kell. The Kell appoints an Archon (the guy you see on the first mission). The Archon's main duty is to communicate with the Prime Servitor (the conscious that links all the Servitors together). The Fallen live off a substance called Ether, which the Servitors produce. The other ranks are Captain, Vandal (specialist soldier) and Dreg (who are Fallen who have had 2 of their 4 arms removed, until they have earnt the right to regrow them). There are also shanks who are like scouts.