The Mega-Weapon is an extremely powerful weapon in the world of Ninjago.
The Mega-Weapon has the form of a large staff, on top of which a large, asymmetrical disk is affixed. The disk has many engravings carved in it, likely showing creation-themed elements.
The makings of the Mega-Weapon were first conceived at the beginning of the world. The Four Golden Weapons of Spinjitzu had existed since before Time had a name and, used by the First Spinjitzu Master, created the world. The Weapons went through many legend-making events, until, in an attempt to destroy the Great Devourer, they were stolen by Lord Garmadon. After destroying the great beast, he fled to the ruins of the Destiny's Bounty. Once there, he resurrected the remains using the Weapons, renaming it the Black Bounty. Then, he convinced the remnants of the Serpentine to follow him. Having an army behind him now, he flew to the Golden Peaks, the birthplace of the Four Weapons.
Once arrived at the Golden Peaks, Garmadon went through an ancient ritual that fused the Four Weapons into one. Having now the ultimate weapon in his grasp, he left the Golden Peaks, and to start planning his battle strategies. However, he accidentally activated the weapon, creating the original owners of the Black Bounty: Captain Soto and his pirate crew. This drained most of his energy. Captain Soto and his crew captured the weakened Garmadon, along with the Serpentine, locking them in prison cells in the hold of the ship.
After being accidentally freed by his son, Lloyd, Lord Garmadon took back the ship, and flew away. He created a squad of four Bizarro Ninja to attack the Ninja, but to no avail. In another attempt to stop the Ninja from winning the Ninjaball Run, he created a massive chasm in the ground, but again to no great gain. Next, he resurrected a Grundle, an ancient prehistoric monster. He was again thwarted. Lastly, Garmadon attempted to create a portal through space and time, and destroy the past, thereby destroying the Ninja. The Ninja followed him into the past, and destroyed the Mega-Weapon, setting things straight.
The Mega-Weapon was capable of creating anything the user's imagination could think of. This was demonstrated by Lord Garmadon's creation of rogue Ninja, pirates, and other objects.
As also demonstrated by Lord Garmadon, the Mega-Weapon can only by used effectively once per day, as it's use saps the user's energy. As a result, Lord Garmadon was only able to create 4 different objects over a period of 4 days.