Megatron, also known as Galvatron whilst in his prototype form, is the main antagonist of the Transformers Cinematic Universe. He is an extremely powerful, destructive, and warmongering Decepticon hell-bent on ruling over humankind and the Earth. His cause has led to him come in conflict with his former brother-in-arms and archenemy, Optimus Prime.
He was a former student of Sentinel Prime, appointed to be Cybertron's Protector and commander of its defense force. But Megatron resented his brother, knowing he was a Prime and therefore Sentinel's favored student. This anger allowed the Fallen (the true founder and leader of the Decepticons) to ensnare him and led to the revival of the Decepticons. Following the demise of both the Fallen and Sentinel Prime respectively, Megatron was brought back to life thanks to the machinations of Cemetery Wind, KSI, and Lockdown, and eventually teamed up with Quintessa to revitalize Cybertron by destroying the Earth (which were, in reality, Primus and Unicron).
He is voiced by Hugo Weaving in the films and currently Frank Welker.