But absolute power corrupts absolutely, and Megatron's Decepticon revolution, like many such movements, ended up becoming a whole new tyranny. Powerful, charismatic, violent, and full of rage for any who would stand in the way of his ambition and drive, Megatron brought Cybertron past the brink of destruction. Now he and his former brother-in-arms Optimus Prime fight on other worlds. Megatron will never forgive Optimus for "betraying" him, stealing from the Council the rank of Prime which was rightfully his. He has reserved for his old friend the greatest honor possible: glorious death at his hands.
Megatron has been gone for three years, but now he's returned. He's brought the monstrous Dark Energon to Earth, and quite possibly left his sanity in space.