The Fallen, formerly known as Megatronus Prime, is the true leader of the Decepticons and Megatron's master. He is the titular main antagonist of Transformers: Revenge of the Fallen.
Thousands of years ago, the first Cybertronians, known as Primes, landed on the planet known as Cybertron, and spawned the race known as Cybertronians. However, one Prime, known as the Fallen, defied an important rule to respect all life and caused a war that destroyed Cybertron.
The Fallen constructed the Solar Harvestar, a device that could destroy suns and create energon, the life-blood of all Transformers. As the Matrix of Leadership was needed to activate the Solar Harvestar, the Autobots hid it in a tomb made of the Primes body. The Fallen was a skilled and powerful warrior, and slaughtered every Prime except an orphan, Optimus Prime, who was hidden away.
The Fallen came into contact with Optimus Prime's brother, Megatron, and manipulated him into reforming the Decepticons and instructed him to steal the All Spark. However, Megatron was killed when a young teenager named Sam Witwicky shoved the All Spark into his chest, apparently destroying it.
Two years later, Megatron was revived by the Constructicons and flew to the star ship, Nemesis. There, he met Starscream in the ship before meeting The Fallen. The Fallen revealed to him that the All Spark was only a vessel and is in the mind of Sam Witwicky. Megatron said that Optimus guarded him, and the Fallen explained that only a Prime could defeat him. They formed a plan to capture Sam and lead Optimus too them, where they woul kill him and leave no one able to stop The Fallen.
Megatron, Starscream, and Grinder abduct Sam and are about to remove his brain when Optimus and Bumblebee burst into the building as planned. Megatron is shot out of the building and he fights Optimus in the forest. He has the disadvantage until Soundwave and Grinder aid him and Optimus is beaten down and shot through the forest. When Megatron asks if the survival of their race worth a single human life, Optimus says they'll never stop at one and viciously attacks, cutting off Starscream's arm and killing Grinder by ripping his race apart, but Megatron stabs him from behind and kills him. The Autobots arrive and Megatron and Starscream retreat.
As the last Prime is dead, The Fallen rises and leaves the Nemesis, and revealed himself to the humans, telling them that if they didn't turn over Sam, the Decepticons would destroy Earth. Instead, the Autobots and Military battled the Decepticons in Eygypt while Sam searched for the Matrix of Leadership. However, when he found it, it turned to dust.
The Fallen sends Devastator to destroy the pyramids surrouning the Solar Harvester, but he is detsroyed by a destroyer ship. At the same time, Sam has a vision of the Primes and the Mtrix puts itself together. Sam uses it to revive Optimus Prime. However, the Fallen appears and pins Optimus to the ground while he steals the Matrix and disappears just as quickly as he had appeared.
The Fallen destroys the military and activates the Solar Harvester. Fortunately, Jetfire gave his parts and spark to Optimus, making him more powerful than before. He flew to the Solar Harvester and shot it, blowing it up. Optimus and the Fallen crashed to the ground, and The Fallen told Optimus to die like his brothers but was blasted away while Optimus exclaimed that they were his brothers as well. Megatron jumped on Optimus but his own hand cannon was turned on him and blasted himself in the face before being shot through a wall by Optimus.
Optimus and The Fallen battled. And Optimus speared The Fallen thjrough the mouth. The Fallen ripped off one of Prime's cannons and hit him with it, but Optimus ripped off The Fallen's face, saying that he rised while The Fallen falled. Megatron and Starscream retreated.