Megatronus

If the first Transformers were Primus' disciples, then The Fallen is his Judas. It is said that At the dawn of time, he was Megatronus, one of a brotherhood of vastly powerful beings dedicated to the well-being of their world. But according to legend The Fallen would gain his current moniker by turning against his brother Primes and his purpose to pursue his own ends, also earning the moniker "the first Decepticon" in the process. His frightening form is alleged to be a metal cage for primal, burning forces of chaos, giving him the appearance of a living furnace. These cauldron-like forces purged him of what empathy and morality remained; all that remains now is hatred and purest rage.

The Fallen is immensely powerful. His acolytes claim he commands mystic, entropic arts, and when at full strength he can un-make creation at his focused will. He is rarely defeated; at best he is contained, where he waits with eternal patience for the chance to unleash his dark powers once again. Though it is rumored he has at times been imprisoned between dimensions, this rarely lasts. When his power is at its height The Fallen can open space bridges and travel between dimensions and realities at will.

If there is any reprieve, it is that there may be only one of him. Like his brothers, The Fallen is considered a multiversal singularity, meaning that in all of the vast multiverse, he has claims to have no alternate-universe doppelganger. He may appear different in different realities but that is merely surface; it is always the same singular force of eldritch evil beneath. In several realities he is the originator of the Decepticons, the catalyst that prompted Megatron to claim power for himself rather than the greater good. While the exact details of his past are said to be complicated beyond the understanding of mortals, two things remain consistent: He is bad news, and he's on fire.

OOC Note
On the MUX, The Fallen is just a powerful dude with a scary reputation.