I think it's like this: Hive = Titan Fallen = Hunter Vex = Warlock My reasons for Hive = Titan are that the Hive's main leaders seem to be hive Knights, this come closest to the Titan class. They're both heavy infantry of their own armies. Their abilities and weaponry are similar: Wall of Darkness = Barricade. Fist of Darkness = Fist of Havoc. Cleavers = Crown- Splitters, hell, even the way Titans run with the Crown-Splitter is quite similar to how Knights run with a cleaver. My reasons for Hunters being Fallen are honestly very easy to explain. They both have cloaks, and can go invisible. Warlocks and Vex share one thing in common, the knowledge of time and space. Warlocks can blink, Vex can teleport.