The Mega Weapon is a weapon evoloved from all four of the Golden Weapons. When Lord Garmadon took all of the weapons he went to Golden Peak to creat the Mega Weapon. Apparently it doesn't have the power destroy instead it has the power to create. Apparently Lord Garmadon had to kill Princess Bubblegum in order to steal the Golden Weapons.
He was the one that forged the Mega Weapon from all four Golden Weapons
She made a special pair of gloves that would allow her to control the weapon
Lord Garmadon granted him control of the Mega Weapon if he would do his bidding