Im holding my theory this "shapeshifting" final boss is the speaker in a new form. He sabotaged our city and allowed a full on assault and took the form of the consul, to allow the warlord Ghaul to overthrow the emperor, as the previous one probably had no intention of invading us. Why? Because the light and the dark are the same thing with a different name and face, they allow whoever touches it to be immortal, and whoever doesnt to want to be immortal. Entropy is the decay of everything living, essentially everything we have been told by the speaker has been fabricated to make us war with enemy races just like the hive are taught to hate the light, the fallen are trying to get their Great Machine back (like us in D2), and the cabal have been promised it as a birthright. The powers we use are not what they are made out to be, we are lent this power on the condition we kill our enemies; a bit like Oryx and Akka but just with extra steps. Now you've read this i think you are going to be in denial. But we have all been told to fight an enemy that "threatens" us that we barely understand. Rasputin? Hes an AI created by Golden Age technology, evil. Vex? Burrowed into planets terraformed by the traveller to simulate every possible outcome to assure its plan never fails, evil. All of this is for one purpose, get every species to fight, eliminating the weak and leaving the strong. We will realise this and probably ally with every other race, or atleast the Fallen. And perhaps help the hive to lift their curse. The cabal dont have any magical powers like everyone else, they are just a military force.