There is also a Hive with its "soul" escaping in that concept. Concept art is concept art. I personally think Fallen are Synthetic lifeforms and the mist is an essence that brings their body to life, though Bungie has stated in the past that it is "technically" the gases of their native planet escaping their suit. If you are really interested you should read the Grimoire cards, I think they explain the Cabal. As for the 5th race, Bungie has said little about them and they will probably be the main campaign theme for Destiny 2, Destiny one + DLC explains the world, Destiny 2 adds the aforementioned conflict. (They have concept art in the installation screen, so don't even try to tell me they are a dropped concept.)[spoiler]"Sence"? Really?[/spoiler]