The Fallen was one of the first 13 Cybertronians created by Primus. This group was known as the Thirteen Primes, and each member was given a specific aspect of reality to look after. The Fallen was chosen as the guardian of entropy.
Eventually, the Fallen joined Primus' evil brother, Unicron, and became his first Herald. After a great battle, the Fallen was defeated and locked away by his fellow Primes.
At some point, he recruited two brothers: Xaaron and Xintun, and commanded them to gather beings for him to feed on. When the Fallen only required one more being in order to gain enough energy to leave his prison, Xintun attempted to trick the rogue Makuta Ynot into entering the dimension, and when that failed, he chased him across the universe, and finally managed to force him to enter the dimension. However, Ynot used a mask to transform Xintun into a Hollow, forcing Xaaron to kill Xintun by giving him to the Fallen in order to feed on. The Fallen promptly emerged from the dimension, only for his body to be destroyed, revealing his actual form, a being made completely of fire, leaving him angry, and seeking revenge.
Of course, none of this actually happened because of Blackout rewinding time by 6 months using the Vahi Cube, but given the fact that the Fallen exists outside of normal space and time, he still remembers, just like everyone else. Ynot had the Toa Idiotas search out Xaaron in order to summon the Fallen to aid him in the fight against Blackout. Later, he led the way during Ynot's attack on Darkmount, and got stabbed with the Master Sword and blasted with lightning. At that moment, he managed to look into Blackout's mind, and found out that Impel Down would be Blackout's next target. Unfortunately, he died before he found out more, but managed to tell Ynot to protect Impel Down, "or all of this will have been in vain.....".
Except he didn't actually die, and sent other Heralds out into the universe to retrieve four objects which were required for the resurrection of Unicron: an orange sphere, a green cube, a blue pyramid, and Starscream's antidermis. Once Unicron had returned and began to attack BZPower, The Fallen tried to absorb a distortion in space, only for demons to emerge from said distortion and attack him. The Fallen then teleported away and unleashed his ageing effect powers on a building where Zev, Namah, and Ryoko were.
Then Nadle attacked him. And then Primus grabbed him, shouted at him and threw him at Unicron, leaving him damaged and floating in space. Despite this, he was apparently fine for the Battle at the Omega Point, which he and Vector Prime ended by locking the Nobodies and Organization 13 members in another dimension.
Following the destruction of Unicron, the Fallen took control of the remaining Heralds, and retreated to an unknown location. He also created a plan to bring about something called "Terminus", which required Angolmois to function properly. He had his minions Kranz and Entropy watch Caiaphus, Starscream and Ricochet respectively, while he himself watched over Blackout, who was apparently vital to his plan.
When Lord English was freed from his prison, the Fallen found this troubling, and considered whether to join in the effort to fight him off... at which point Entropy arrived and told him that English had appeared. This angered Fallen very much, causing him to break the fourth wall.
- For more information on the Fallen that inspired this Fallen, read this.