Megatron is the leader of the Decepticons, a race of evil sentient machines from the planet Cybertron.
Megatron was created millions of years ago along with several other decepticons, and quickly rose to be their leader, his particular brand of maliciousness a perfect fit for the role.
When they first started to attack Cybertronian cities, rumours began to spread among the peace loving Autobots. However, one individual, Orion Pax, refused to believe them and on encountering Megatron offered him the hand of peace and friendship.
Megatron appeared to accept this and acted normally, until Orion allowed him access to the stores of energon cubes he had where he worked. Megatron revealed his true nature, and attacked and almost destroyed Orion, who was taken in by the engineer Alpha Trion.
While his forces were recovering the energon, Megatron encountered an unknown robot bearing the Autobot symbol. Upon asking his name, Megatron came to know the being that would be his ultimate foe for millennia, Optimus Prime.
Megatron and several Decepticons followed the Autobots from Cybertron when Optimus Prime and his friends fled in their ship the The Decepticon ship, the Nemesis, followed and attacked during the flight.
The battle damaged the ships systems and caused them to crash, along with both the Autobots and Decepticons on an unknown plant called Earth. Millions of years later a volcanic eruption caused the Arks computer, Teletran-1, to reactivate one of the Decepticons who in turn reactivated the rest of his cohorts.
They went on a destructive spree, claiming as much energy as they could, but after Starscream blasted the rocks around the fallen Ark as a ‘leaving present’ to the Autobots, Teletran-1 was able to reactivate Optimus Prime and his colleagues, thus the Transformer civil war continued on Earth .
Megatron hates all forms of Organic life, finding them all inferior and disgusting to his mechanical form.
He cares for nothing but himself and his quest for power, and has repeatedly left his fellow decepticons to fend for themselves when damaged or captured by the Autobots.
Megatron’s hatred extends to Optimus Prime the most, who has repeatedly survived attempts on his life, one on one duel’s that weren’t in his favour and even being shot at the sun. Megatron just can’t seem to kill him, despite his boasts to the contrary.
He despises some of his fellow Decepticons as well, especially Starscream, who has repeatedly tried to usurp Megatron and assume leadership, though Megatron cannot deny Starscream’s usefulness to the cause and so will not destroy him.
Megatron’s original transformation was into a hand held, silenced laser pistol, normally carried by one of the other Decepticons.
In the Series Transformers: Robots in disguise, Megatron was able to transform into several modes: Bat, Two-headed Dragon, Car, Jet and his robot form.
In Transformers: Armada, Megatron was a tank.
In Transformers: Energon, Megatron was a futuristic jet.
In Transformers: Cybertron, Megatron was a futuristic car.
In the 2007 movie, Megatron was a Cybertronian jet.
In the Beast Wars series, Megatron was a T-Rex, who later became a full robot and jet for Beast Machines.
In a final battle that resulted in the death of Optimus Prime, Megatron was severely damaged, prompting Starscream to jettison him into space, where he encountered the monster planet Unicron. Unicron reformatted Megatron to become Galvatron, who’s original form was a mobile gun platform. In almost every subsequent iteration, Megatron has become Galvatron, though his transformations have rarely changed.