note: you can use him in your fanon if you like
Galvatron is the result of Soundwave using a body-modifier on the dead body of the Unicron posessed Megatron. he was fierce when he heard that the Autobots made peace on Cybertron and not to mention the resurrection of his mortal enemy, Optimus Prime (hey this is my fanon i can do what i like). in order to deal with this, he refounded the Decepticons and transported the Autobots (and their Cybertronian base) to Earth. ready to destroy the planet in the view of the Autobots, he made a deal with Unicron: to make Cybertron his new body so he can devour Earth.