There was a time on Cybertron, in the twilight hours of the Golden age, when Optimus and Megatron were not sworn enemies. Instead, Megatron was in the gladiatorial combat frankly because he enjoyed that sport. Being a champion, he took the name of the legendary Megatronus, one of the Thirteen Primes that betrayed his comrades and became the first decepticon. Megatronus (Megatron) vowed to challenge Cybertrom's leadership and demand that all Cybertronians be treated as equals. This gladiator turned revolutionary rapidly to gather loyal following. Before long, Megatron appeared before the High Council to propose his vision a just society. And it was there that he began to reveal his true colors. Proclaiming the need to overthrow the old guard with force and arrogantly demanding to be named the next Prime. But Orion did not believe in violence as means of achieving justice. The sparks and minds of the Council were moved by Orion's words. Here, for the first time since Cybertron's Golden age, stood someone worthy of being a prime. But that honour could only be truly achieved...By earning the legendary Matrix of Leadership. His ambitions thwarted, Megatron spitefully severed all ties with Orion and the Council. And came to wage war on all who oppossed him through his growing army of followers which he named "Decepticons".
He vowed to claim the Matrix of Leadership himself, wherever it might lie. In time, warfare consumed Cybertron, poisoning the planet to its core.
(If you guys see a problem here or it doesnt make any sense from because of the prologue, please inform me so I can fix it)