War for Cybertron has ended almost 20 years ago and Autobots and Decepticons created "Cybertronian society" and members of this society were leaders of every fraction: Optimus Prime - Autobots, Megatron - Decepticons, Grimlock - Dinobots (not really a fraction more of a team), Soundblaster - Mercenaries, Metroplex - Titans, Override - Speedracers from Velocitron, and Alchemist Optimus Primal who represented all beasts(maximals/predacons)
Of course there were many bots who disagreed with this and Starscream was one of them, with his team and help of a strange "voice in his head" he corrupts Allspark and starts destroying everything and everyone, he was so strong with the allspark so even Optimus and Megatron couldn't stop him.
Shortly after Starscream got allspark power the portal appears in the sky and someone one shots starscream in the back...
That "Someone" was actually Megatronus Prime who telepathicly told Starscream what to do to get power, but he actually wanted Starscream to break the power balance and that would create a hole in the universes. Right after Megatronus entered our universe he tried to get the allspark and was attacked by Optimus and Metroplex.
Megatronus instantly kills Metroplex and that shocks everyone. Optimus manages to take the allspark and commands everyone to retreat. Megatronus holds the hole between universes so it wouldn't close and many ancient Decepticons enter through it. They start destroying Cybertron and killing everyone.
