The thirteenth of the Firstforged is known only as The Fallen; his name is erased so thoroughly that no hint or trace of it remains. It is believed that he was intended by Primus to be a "catalyst of change", embodying the central principle of Transformer existence that is the innate ability to change. There is no record of where he demerged or if he had spawnlings. Some Primordialists suspect that The Fallen did have spawnlings, and a full line of descent, but that they were massacred after The Fallen made his allegiance with the Great Devourer.

The Fallen allied himself with the Great Devourer and attempted to bring the chaos god to Cybertron, but was prevented. He was then banished to a place not described clearly in any record.

During the late Golden Age and early days of the Third War, some Decepticon speechmakers claimed that the lack of The Fallen's programming from modern Cybertronians was the cause of the stagnation and inflexibility under Autobot rule. These sentiments have since been heavily repudiated by the Decepticons. Unsurprisingly, at least six Autobot demagogues have accused Megatron of being The Fallen.

"The Fallen" is a translation of the meaning of his name in Modern Autobot, aimed to recollect the fall of Lucifer. The Fallen's name in Ancient Cybertronian is not recorded; indeed, places where it would have been written are hacked out and burned, to completely destroy his name. In Primaltongue he was called Iarakuhr meaning "the undoing, reversing, gone one, go away from me!" and is more an invocation against his existence than a name. In Modern Autobot he is called Klarkaum, meaning "the enemy who has left". Against the normal rules of grammar, the word Klarkaum is formed from two suffixes with no prefix or root word. Klarkaum transliterates into Murion as Kular Karum. kular, meaning "one of the attackers", was often used by Kaonics as a slur against Primordialist Iaconics. In High Legion he was called Sawocasaboulo, meaning "facing away from the circling motion of his thought". This invokes the concept of a current trapped in a cave, permanently circling but unable to escape, and the rejection of the thought this represents by the Legascions. In Modern Deceticon he is called Ozonkam, meaning "the strange attacker". As with Klarkaum, Ozonkaum is grammatically incorrect as it is formed entirely suffixes. In both cases the words are therefore nonsense words rather than true names.