Megatronus Prime, later known as the Fallen, is one of the original Thirteen Primes created by Primus at the dawn of Cybertron to battle Unicron. Unlike his brethren, Megatronus was inherently drawn to entropy and destruction, a nature that ultimately led him to betray his fellow Primes. In a fit of jealousy and rage, he murdered Solus Prime, a fellow Prime and his former lover, committing Cybertron's first murder. For this appalling crime, he was banished to another dimension and imprisoned.

In the series Transformers: Robots in Disguise, the Fallen serves as the primary antagonist of the first season. His followers, including the Decepticon Steeljaw, work to construct a portal that will free him from his dimensional prison. Once released, the Fallen reveals himself to be the founder of the Decepticons and the master of Megatron, though he is distinct from Megatron himself. He orchestrates the crash of the prison ship Alchemor on Earth, freeing hundreds of Decepticon inmates to cause chaos and further his own agenda.

The Fallen is characterized by immense pride, a vengeful nature, and a deep sense of grievance against both Cybertron and Earth, which he blames for his exile. He is manipulative and ruthless, using his influence to direct Decepticon forces from behind the scenes. In battle, he possesses superhuman strength and durability, wields the dangerous Requiem Blaster, and can call upon dark science and gravitational powers. His face serves as the direct inspiration for the Decepticon insignia. Despite his power, he is ultimately defeated when Bumblebee, Sideswipe, and Strongarm combine their Decepticon hunters to fire a powerful energy beam that seemingly destroys him. His role in the story highlights the ancient origins of the Autobot-Decepticon conflict and tests the leadership of Bumblebee and his team.