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Transformers: Generation 1
Megazarak is the Japanese name of Generation 1 Scorponok; he was also featured as Majin Zarak.
In this Transformers: The Headmasters, Megazarak is the larger body for the Destron robot Zarak. He was initially depicted as Zarak, a shadowy figure, shrouded in darkness, until he completed construction on the Megazarak "body". He even assumed command of the Destrons for a while when Galvatron was thwarted by the Cybertrons. When Galvatron was killed during a battle with the Cybertron Headmasters, Megazarak took back the leadership of the Destrons. At the end of the series the Megazarak body was destroyed by Fortress Maximus.
In the Beast Wars Second film Beast Wars Second: Lio Convoy's Close Call!, Galvatron's attempt to summon Beast Megatron ended up bringing a being known as Majin Zarak to him instead. This tyrannical figure soon turned against Galvatron, but was eventually defeated by the combined efforts of Lio Convoy and Beast Convoy. "Ask Vector Prime" later revealed that this Majin Zarak had once been a version of Scorponok who, attempting to harvest power from across time and space, was consumed by the energies he gathered and fused together with his floating fortress. Going on a spree of destruction, he was eventually evicted from the planet by the combined forces of the Autobots and Decepticons, after naming him after an ancient Predacon named in the Covenant of Primus. He continued his destructive tendencies in space before being summoned to Gaia, where he is thought to have met his end. 
Megazarak is the Japanese name of Superlink/Energon Scorponok.
One of the Heralds of Unicron, Megazarak comes from a dimension where he has all but destroyed the Autobots and is the ruler of the Decepticons.
This version of Megazarak was noted for his insatiable appetite for destruction and mayhem outstripping that of even his predecessors. Megazarak soon nearly killed the Prime of that reality (later that individual would be taken by Unicron and reborn as Nemesis Prime), and nearly annihilated the Autobots once and for all. The rest of the Decepticons quickly learned to both fear and respect their new master, and he soon assembled his own personal strike force -including Sunstorm, Dreadwind, Smokejumper, among others. His Mini-Con partner, Caliburn, actually loves the level of carnage his partner causes. Megazarak seems to have no known weaknesses, but he does have a preoccupation: he seeks to destroy Defensor, the only Autobot to have escaped his wrath. Megazarak was released as a special edition figure, along with Sentinel Maximus available at BotCon.
Megazarak's only fictional appearance came in the BotCon Transformers: Universe series, where he was shown seeking out the Oracle (of Beast Machines fame) to help track down the last Autobots on Cybertron. He was confronted by Ultra Trion, who valiantly attempted to hold him off, but Megazarak killed him with his main cannon. However, Trion had his victory even in death, as he had bought enough time for the Oracle to be moved.
"Ask Vector Prime" later revealed that there were two Megazaraks, both of whom hailed from a version of the Robots in Disguise universe. While one traveled to an Aurex reality-a variant of the Unicron Trilogy reality-the other remained home to ravage his own native universe, taking over the Decepticons after the death of Megatron. Among his many atrocities were using a Robo-Smasher to turn the Build Team into the Constructicons and Team Bullet Train into the components of Dominus Trannis, poisoning Scourge and turning him into Toxitron, and creating Megabolt from the remains of Megatron. He also destroyed many of the Autobots, including all the members of the Protection Team except for Hot Spot, who later received the Matrix of Leadership from Ultra Trion and became Defensor. He slaughtered the remnants of the Autobots after both they and he lost fighters to Unicron's machinations, but was later defeated himself by Defensor and the surviving Predacons under Sky-Byte.
The other Megazarak was responsible for the fate of the Optimus Prime who became Unicron's Nemesis Prime, and leader of a faction known as the Destructicons that also included Jhiaxus. After discovering a reality in which Cybertron was racked by a three way war between an oilthirsty Optimus Prime's Autobots, Rodimus' Omnicons, and Megatron's Decepticons and had been attacked by Primacron's Mini-Cons, he left Jhiaxus in charge of their home reality-where only a small group of Dinobots continued to fight against them-and made his way to this one with a small force. The Destructicons and Decepticons formed an alliance, but after crushing their opponents Megazarak turned on his "allies" and wiped them out as well. Unicron then recruited the Destructicons for his own schemes, leaving the reality to be virtually destroyed by Tornedron. This Megazarak subsequently became known to the Defensor who had defeated his counterpart, and the Autobot leader set out to rid the multiverse of a second tyrant. 
- Universe Megazarak with Caliburn (2004)
- In the Transformers: Universe line, Megazarak is a repaint of Armada Megatron (with lights and sound electronics removed).