The Fallen is one of the Thirteen original Primes who betrayed his brothers. Most audiences know him as the main villain of Transformers: Revenge of the Fallen (2009).
The myth of Megatronus
In the mythology of the Thirteen, the Fallen was once called Megatronus Prime. He envied his brothers and craved power, which eventually led to his exile and curse. Megatron's name later derives from his, and in some versions Megatron deliberately chose it for himself.
In the 2009 film
In Revenge of the Fallen the Fallen is Megatron's master, imprisoned in space. He once broke the Primes' chief rule — never destroy worlds that harbour life — by trying to activate a machine on Earth that drains the Sun's energy. Now he wants to finish the job. He was voiced by Tony Todd.
Why he matters
The Fallen shows that betrayal is built into Transformers history from the very start: before Autobots and Decepticons, brother turned against brother. That makes the war of Optimus and Megatron part of a far older tragedy.
