Galvatron is the recreation of Megatron. Once his new body was complete, Unicron gave him the Sweeps, a group of deadly robots. They went back to their home planet were Starscream was about to be crowned the new ruler of the Decepticons. Galvatron came and killed Starscream in front of the others, thus regaining his control of the Decepticons. With Optimus Prime out of the way he was concerned about retrieving the Autobot Matrix of Leadership for Unicron. He found the Autobot that was carrying the Matrix. Ultra Magnus tried to stop him, but the Matrix would not open for him. Galvatron then went back to Unicron and Unicron turned on him. Unicron swallowed both Galvatron and the Matrix up. It wasn't until a group of Autobots including Hot Rod (Rodimus Prime), Kup, and Arc ee came in to Unicrons body to stop him from devouring their planet, Cybertron that the Matrix was opened and used to destroy Unicron. Galvatron escaped being destoryed in the explosion as he was hurled from his creator's body by the Autobots' new leader just beforehand.
Major Story Arcs
In Marvel's continuity, Unicron made his way towards Cybertron over a decade earlier than in the animated movie. Seeking a powerful agent to weaken Cybertron before his arrival, he had several of his servants abduct the Galvatron of an alternate Universe, one where Unicron had destroyed Cybertron and the Decepticons annihilated most of the Autobots, including their leader. This alternate Galvatron was greatly displeased at being plucked from his world of triumph, and soon took the opportunity to betray his new master.
While leading an attack on Cybertron, Galvatron snuck away from the rest of Unicron's forces and aided Xaaron, leader of Autobots on Cyberton, in awakening the Transformers' creator deity and Unicron's nemesis Primus. Not having the power to face Unicron directly, Primus summoned all of the Transformers to Cybertron where they could hopefully combine their forces and destroy the planet-devourer. However, upon his arrival, Unicron's size and power completely terrified the Cybertronians, and most fled for their lives. Xaaron (now possessed by Primus) and Galvatron were the first to attack Unicron, and although their assaults caused little physical harm, they did inspire the rest of the Transformers to fight back.
In the end, Optimus Prime sacrificed himself and used the Matrix to destroy Unicron. Cybertron was left on the brink of destruction, forcing the Transformers to evacuate their homeworld. Galvatron stowed away on the ship stolen by Starscream and Shockwave, and was shocked to encounter Megatron on board. Galvatron attacked this alternate version of himself in a bout of madness, before coming to his senses and considering an alliance instead. This idea proved short-lived as Ratchet, the only active Autobot onboard crashed the ship into the Earth. Galvatron was the only one to survive the crash, although it left him badly injured and in this state he was overpowered by Fortress Maximus and left inoperative at the bottom of a frozen lake.
Galvatron hails from the Golden Age of Cybertron. He was chosen to be among the crew of the first Ark, which was lead by Nova Prime, for his willingness to throw himself into the unknown as Nova had planned on exploring the Benzuli expanse. However, the venture would end in failure as all efforts to probe the expanse had failed and so Galvatron would sabotage the ship and drove the ship into the expanse, trapping it and the crew in the dead universe that lay beyond it. Here, in the dead universe, Nova Prime was consumed by the darkness - transforming him into Nemesis Prime - and Galvatron would become his herald but only because of his command of the darkness. Years later, Galvatron would do the bidding of Nemesis Prime, with help of the powers of the darkness he possessed, by clearing the Benzuli expanse of any being who tried to get near it and would retrieve Thunderwing's body, which lay dormant since the Autobots took him down after he was revived.
Galvatron would go to Earth, under Nemesis Prime's orders to stop the Reapers from destroying the Earth as it would interfere with the expansion of the dead universe and prevent its denizens from returning. To this end, he was given the touch of unlife by the darkness Nemesis possessed and he would use it to turn Deathbringer into a harbinger of death, whose touch disintegrated everything, causing him to destroy his own fellow reapers, and saving Earth for the moment.
Galvatron, along with the rest of the denizens of the dead universe, return to the core universe they came from to begin the expansion. Galvatron leaves Cyclonus to take on an Autobot vessel by himself while he heads to Garrus-9, where he observes the confrontation between Nemesis Prime and Optimus Prime. During their confrontation, the darkness that Nemesis Prime possessed passes to Optimus, as it seeks to pass from Prime to Prime just like the Matrix. Seeing that Nemesis Prime no longer had the darkness, Galvatron saw no reason to serve him anymore so while Nemesis Prime was focused on trying to kill Optimus, Galvatron shot Nemesis Prime in the back, killing him. He then approached Optimus and told him that if he didn't want the darkness to take control of him, then he could either hurl himself into the nearby solar pool and kill himself, or give the darkness to him. He then tells Optimus how he heard the darkness speak to him and promise answers to riddles of who he was and what he was destined to be but how it ultimately couldn't resist the tug of the matrix vacuum inside of Nova Prime. Optimus ultimately gives Galvatron the darkness, but then he grabs Galvatron and tosses him into the solar pool, seemingly killing him.
All Hail Megatron
Galvatron emerges from the solar pool that Optimus hurled him into, alive, and now wielding the power of the Darkness once held by Nemesis Prime. He approaches Cyclonus and stops him from killing Scourge, who tries to tell Galvatron that they are dying and have no reason left to live and that only by killing them will they get their honor. However, Galvatron tells him that he’s going to join him and that he has a mission for them. He uses his newfound powers to then restore Scourge to full power and then creates an army in his [Scourge‘s] image. He calls them the Sweeps and tells Scourge that they are bonded to him and that if he keeps them in line, he will be a warrior king. Cyclonus then begins to protest against Galvatron because he does not want to be turned into a warrior slave, but Galvatron then tells him that his spirit refuses to die and that he was able to survive out of the dead universe longer than everyone else because of it. He then gives Cyclonus strength and then makes him his lieutenant. After accepting his position, Cyclonus asks what their mission is, and Galvatron responds by telling him how they were once explorers and now that mission is over and they have learned that the only universal force is change, and that they are now part of that force.
When Megatron turned into Galvatron, he became far more ruthless and evil than before, which makes him even more fearsome in the eyes of his enemies. Arrogant and power-hungry, Galvatron seeks to destroy the Autobots and conquer everything.
Powers and Abilities
Being an upgraded version of Megatron, Galvatron possesses massive strength and power, enough to lift far more than 2000 tons and to pulverize an Autobot into scrap metal with sheer, brute force alone. He is also nearly indestructible, and only the most powerful forms of blunt trauma and artillery could ever hope to harm him. In robot mode, his particle cannon carries a laser that emits chemically- produced, direct-current electricity. With Unicron at full strength and as an ally, Galvatron's cannon mode possesses far more power than any other Cybertronian that has ever existed.