The original name of the character known as The Fallen is Megatronus. In Transformers lore, Megatronus was one of the original Thirteen Primes, the foundational Cybertronians created by Primus in several major continuities.
The name is closely tied to the character’s fall from the ranks of the Primes. In versions such as Transformers: Revenge of the Fallen and Transformers: Robots in Disguise, Megatronus betrays his fellow Primes and becomes a destructive, exiled figure. The title “The Fallen” describes his betrayal and disgrace rather than being his birth name.
Continuities vary in detail. Some call him Megatronus Prime, while others use Megatronus alone; the 2009 film emphasizes him as a rogue Prime and mentor to Megatron. The name also explains Megatron’s own name: in some stories, the Decepticon leader adopts or shortens “Megatronus” as a revolutionary identity. The Fallen should not be confused with Prima, Vector Prime, or Alchemist Prime, who are separate members of the original Thirteen.