The Decepticons started acutally with one Cybertronian, was one of the original seven Primes, his name was The Fallen, who was formerly known as Megatronus Prime. When the original Primes found the AllSpark, they realized they needed a power source for it, for as the AllSpark created life, it too, did not have unlimited energy. The Primes would eventually figure out that in order to power the AllSpark, they needed to harvest Energon, from Suns. The Primes, however, had one rule, never. Harvest Energon from a Sun with an Inhabited planet, Megatronus dispised this rule, believing that Cybertronians were the ultimate race, this combined with his hatred for the human race, led him to choose to ignore this rule, he gathered up a large group of like-minded Cybertronians and named them Decepticons, (fun fact, the Decepticon logo is based upon the fearsome visage of the Fallen), this is why Megatronus is also known as the Original Decepticon. Thousands of years ago Megatronus and his Decepticons then start to construct a Star Harvester on Earth, in prehistoric Egypt, and Killed any humans who discovered the Harvester. The Primes discovered what Megatronus had done and went to Earth to confront him, they launched a full on assualt on the Decepticons. The battle was ferocius, Megatronus fought with the other Primes, and while Megatronus was stronger then his brothers and they couldn't kill him, they imprisioned him and took the Matrix of Leadership from him (the Matrix was also the only way to activate and use a Star Harvester). They then sealed the Matrix in a tomb made of their own bodies, their ultimate sacrifice to save Earth, Megatronus would forever more be known as The Fallen, and the Decepticons would then disband and the name would never be used or heard of again, until many years later.
Many years have passed on Cybertron since Megatronus's dreadful actions, Cybertron was at peace, and their Leader Sentinel Prime had just stepped down to let the future actions of Cybertron be determined by the Cybertronian science division known as Autobots, which was led by a Cybertronian scientist named Optimus, and the Cybertronian military, which was led by Megatron. Optimus and his science division were at an expedition digging for old Cybertronian Relics linked to their past when Megatron came flying towards them and asked Optimus how the dig was going, Optimus stated that it was fine and that they found a strange relic in the ground that could be linked to their past. Megatron ordered it to be dug out and returned to his quarters Optimus insisted it be taken to the Science institution for study, and just as Megatron and Optimus were about to argue, they saw the city was under attack from an unknown alien race who sought the energy of the AllSpark, Megatron ordered that the Science division move out of the Expedition and flew off to deal with the invaders. When Megatron arrived at the scene, Bumblebee and Cliffjumper were on the defensive while guarding the AllSpark, Megatron critizied them for not going on the offensive, whilest speaking he was shot in the shoulder by a tank, he dealt with it quickly when Starscream came to ask if he was fine. Megatron told Starsream he was fine and when Starscream asked about Bumblebee and Cliffjumper and what they were supposed to do, Megatron say "to stay out of our way", after the battle Megatron limped his way to his quarters, as instructed the artifact the Science division discovered was in his quarters, as Megatron grew more weak he started to fall, when he fell for what was thought to be his last movement, it was in front of the artifact, but this artifact was strange for it started to speak to Megatron. The first thing Megatron heard was a voice in his head telling him to wake up, the voice repeated this and Megatron rose, demanding to know who the voice, the voice first asked him who he was, Megatron told the voice "I am Megatron, defender of Cybertron, and no one-" Megatron was cut off when the voice spoke to him, when Megatron looked at the artifact it had a strange visage on, the voice spoke to him and explained who he was. The voice had come from the artifact, and the artifact was the very prision in which the original Prime had Imprisioned Megatronus, who was now known as The Fallen. The Fallen explained all the history of him, the Primes, the Star Harvester, and more, the Fallen told Megatron that he needed to find the Star Harvester which he had constructed so long ago and that Megatron would help him find it, and in return, he would grant him tramendous power and rule over Cybertron, Megatron accepted and became his apprentice. Now the Apprentice of The Fallen, Megatron decided to launch a full-scale attack on the alien ships that flew over their planet. With the entire Cybertronian military at his command, Megatron led the attack and triumphed. Optimus questioned the reasoning behind this and went to Megatron's quarters to find some answers. when Optimus, Jazz, and Prowl entered Megatron's quarters, they found nothing, and when Optimus found the artifact, the visage was gone, but the symbols on the side weren't. Optimus said to himself "Megatron must have cleaned it up" before leaving, later, after an anaylsis from Ratchet, they would discover the symbols on the artifact and similar artifacts would have the same symbols, Optimus in turn would have these sybmols, and that they are the symbols of the Primes, thus making Optimus, Optimus Prime. When Megatron returned, The Fallen told Megatron that Optimus had been here, after he wad informed, Megatron decided it was time for action, The Fallen told him "Cybertron will be yours, everything else will be mine". Megatron would later gather Cybertronians who were loyal to him in the city of Trypticon, he stated that they would give Cybertron the rule it deserved and branded him and his follower Decepticons, looks like the Decepticons are making a comback.many more years have passed since Megatron and the Allspark left Cybertron, and both Autobots and Decepticons decided it was time to leave. The Decepticons arrived a while after Bumblebee and HotRod (and by a while I mean a long time), they make their presence known by having Blackout attack a military base in Qatar to find information on the whereabouts of Megatron and the AllSpark, which both seemingly have crashed here. Frenzy searches the files again on Air Force 1 after Blackout's connection was cut by the humans, Air Force 1 lands on the ground and Frenzy meets up with Barricade, who is disguised as a police car. Frenzy tells him they need to find Ladiesman2187, which is the Ebay username for Sam Witwickey. Barricade's attempt at finding the glasses that can lead the Decepticons to the AllSpark and Megatron from Sam is stopped when Bumblebee comes and gets Sam away from Barricade. The Decepticons begin their attack on mission city when Starscream attacks a civilian truck and costs Bumblebee his legs. The second wave of the attack shows Brawl firing at the human military, Brawl unfortunately already suffered a casualty after his arm gets cut off by Ratchet, and suffers a painful death after getting blasted in the spark multiple times by Bumblebee. Blackout arrives on the scene only to be shot in the head not much later by Major Lennox. The ultimate Showdown between Optimus Prime and Megatron begins, and ends with Sam pushing the AllSpark into Megatron's chest, overriding his spark with energy, killing him, leaving only Starscream, who retreats into space to go to the Nemesis, which has crashed on one of Saturn's moons, and there he finds The Fallen on the ship, Starscream sends Decepticons back to Earth while he helps The Fallen build his new army.
Transformers: Revenge of the Fallen (2009)
Two years have passed since Megatron was vansquished by the AllSpark, because of this, only shards of the AllSpark remain. One of the last two shards is stolen by the Decepticon Ravager, and with the help of Long Haul, Scrap metal, Mixmaster, and Rampage, they revive Megatron. Megatron then leaves the Baltic sea to head for the Nemesis and encounters Starscream, and later, The Fallen. The Decepticons arrive at Egypt with more numbers than in mission city, the first sign of their presence is the destruction Starscream causes around the place while the Decepticons look for Sam. When the battle ensuses, the Decepticons were probably at their highest point of power on Earth, but still, Long haul dies (well, his death is never shown, soo..), Mixmaster dies, Rampage dies, ravage dies, Devastator dies, and The Fallen dies, now he joins his brothers, leaving only a wounded Megatron and non-wounded Starscream, and they retreat, not to be seen for two more years.
Transformers: Dark of the Moon (2011)
After the death of Fallen, Megatron again became the head of the Decepticons; Now they have begun to implement plans for the revival of Cybertron. For this, they needed the help of the old Autobot Sentinel Prime, who knew the secret of building a space bridge. Cunningly having achieved his revival, Megatron made a deal with Sentinel. The Decepticons staged a trap and took almost all the columns on the Moon and hid them where they could not be found. The Decepticons began to pursue Sentinel, but soon the time passes to their side. Sentinel opened the portal to the moon for the army of the Decepticons and launched an attack on Washington.
Sentinel brings a message to the UN saying we Decepticons came for your natural resources and demands that the Autobots leave Earth. After having dealt with the Autobots, the Decepticons launched an attack on Chicago. The Decepticons seized Chicago and began installing columns. Then we learned that the Autobots survived and began to fight with them and NEST soldiers. Sentinel ordered all Decepticons to turn on the columns, most of the Decepticons die from the Autobots and NEST soldiers. Part of Cybertron was moved, but Optimus stopped the main column. Sentinel orders the Decepticons to open fire on Optimus, Optimus almost lost the battle with Sentinel, Megatron intervened in the fight of Sentinel and Optimus and accidentally saved Optimus. After the Autobot destroyed the main convoy, the Decepticon ships began to fly away and Cybertron collapses. Megatron offers Prime a remake, but Optimus refuses to kill Megatron and Sentinel in response, and the Decepticons were defeated.
Transformers: Age of Extinction (2014)
Three years later since the Battle of Chicago have not gone by unnoticed, Decepticons fell after the death of Megatron and are on the run. a bounty hunter named Lockdown comes to Earth in search of Optimus Prime, and an organization named Cemetery Wind is hunting down any and all Transformers to find Optimus. But what about Megatron or any of the other Decepticons, what happens to them? well this company call KSI ships all of the dead bodies and starts melting them down and using it to make their own transformers, the first one they make is called Stinger, a replica of Bumblebee, after that they create Galvatron, however, even though they designed Galvatron after Optimus Prime, he keeps looking like Megatron, this isn't a normal bug. You see, Megatron's body may be fully dead, but his mind, not so much. First Megatron's mind was downloaded to computers and had the humans build him a new body (not known to the humans) when they finished building it Megatron infected it with his evil chromosomes, they named the prototype Galvatron, but its really Megatron reincarnated, looks like The Decepticons aren't fully dead. Galvatron has broken from his human chains controlling him and also took control of the other fifty prototypes, including Stinger, with his new Decepticon army, Galvatron leads his army to Hong Kong to detonate The seed, to make an even bigger army. After the battle begins the Galvatron army and Lockdown arrive and fight the Autobots, the entire Galvatron army, Lockdown and Harold Attinger were both destroyed, but Galvatron left saying "we shall meet again Prime, for I have been reborn".
Transformers: The Last Knight (2017)
Three years after the Battle in Hong Kong, as the UN declared the Transformers race outlawed and set up a military organization of TRF to hunt them. The Decepticons began to search for the staff, Barricade informs Megatron that the talisman is in person and offers to make a deal with TRF. Megatron takes two CIA operatives hostage and offers an exchange to release his group of Decepticons - Mogawk, Dredbot, Nitro Zeus and Onslot, except Berserk. The Decepticons began to hunt for Cade Yeager, who has a talisman, the Autobots win, and Megatron orders them to retreat. Barricade pursues Cade and Bumblebee in London, knocking him down. The Decepticons finally get the staff, they activate Stonehenge so that the Quintess begins to pump out the energy of the Earth. Megatron gives the Quintess the staff and the Decepticons began to defend the Cybertron ignition chamber from the Autobots and the people. But the Autobots broke through and most of the Decepticons began to die and were defeated.