If the first Transformers were Primus's disciples, then The Fallen is his Judas. 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 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 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. 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 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.
Like his brothers, The Fallen is a multiversal singularity, meaning that in all of the vast multiverse, he has no alternate-universe doppelganger. In fact, in some dimensions, 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 complicated beyond the understanding of mortals, two things usually remain consistent: He is bad news, and he's on fire.
In the Kingdom Hearts Unlimited Saga, the Fallen is a major supporting antagonist, serving as the true mastermind behind Megatron's involvement in the Blackfrost Organization during the events of "The Chain of Memories". He later returns in The Second Trilogy where he serves as a part-time member of the Nightbreed Organization alongside his master Unicron.