At some point before coming to Earth, Fuse invaded the planet Anur Phaetos, home world of the Ectonurites, the same race as Ben's alien form Ghostfreak. Eventually, the Ectonurites reached an agreement with Fuse; their planet was spared, but they had to aid him in his conquest. Only Vilgax seems to know anything about this agreement. At some other point, Fuse also tried to invade the homeworld of the Glorft, but was repelled. Eventually, Fuse entered our Solar System and invaded Pluto, Saturn, Jupiter, Venus, and Mercury (and possibly Neptune, Uranus, and Mars as well). Fuse now has his sights set on Earth, beginning his invasion of our world.
For most of the war, Fuse acts from behind the scenes, on Planet Fusion. From there, he leads his troops against the Earth's forces and makes his plans and strategies for consuming the world. He has also tried on numerous occasions to study Imaginary Energy, the element that is most effective against his forces. It is not until the mission "Fuse No More (Part 1 of 4)" that Fuse starts to personally participate in the war.
By manifesting a physical form on the surface of the planet, he planned to transfer his energies down to that form, allowing him to appear on Earth, destroy the hero, and power up his forces from inside Fuse's Lair. Fuse eventually managed to materialize himself fully in this form, but thanks to the hero, the shield that protected him was destroyed, thus leaving him vulnerable. In "Fuse No More (Part 4 of 4)," the hero faces Fuse and is able to destroy his form after a long difficult battle. However, because the form in his lair was only an inferior manifestation of Fuse himself, Fuse's energies simply returns to Planet Fusion upon his defeat, and he is able to continue the invasion and his plans as he did before.
After the heroes founded the Academy, Fuse reorganized his forces and created several new monsters to keep a handhold on the war.
Powers and abilities
Most of Fuse's powers is unknown, due to the limited knowledge about him. In battle, Fuse constantly keeps changing his attributes, from Adaptium, to Blastons, to Cosmix, and back to Adaptium. This way, Fuse makes it harder for the player to select the right Nano to fight him with. Over the millenia, Fuse has mastered many different forms of science, magic, and technology as a result of his world-conquering.
Fuse in his incomplete form has the same different powers as his Fusion Monsters, such as stun, corruption attacks, and eruption attacks, but they are much stronger than normal. He is also the only Fusion in the game that can Heal himself. He mainly uses melee attacks and can fire blasts from the limbs attached on his back.
His most well-known power is his ability to create Fusions, evil duplicates of the Cartoon Network Universe's inhabitants, by combining a personal item with Fusion Matter. These efforts, however, drain Fuse of his energy, and thus it takes time for him to replace Fusions that have been defeated in battle.
According to Samurai Jack, Fuse's power pales in comparison to Aku. However, as Fuse prepares to materialize on Earth, Samurai Jack himself openly claimed that all hope would be lost if he did, and that he was unsure if they could win.
Fuse is one of the strongest monsters in the game, surpassed only by Don Doom, Bad Max, Fusion Kimchi, the turrets inside Vilgax's ship, and the Fusion Blowfish, Rudy, Cootie, Judy, and Tootie, all of which possess surprising defensive power.
Shatterguns and rifles are recommended for battle with Fuse, as his more powerful abilities are triggered by moving within a certain distance. Both types of weapons can harm him beyond this distance. Damage and healing Nanos are also recommended, as Fuse's stunning and healing abilities in the first and second forms, respectively, can make overcoming him difficult. Specific nanos would be Leech, Health, and Freedom, though the latter nano is only recommended for the first two phases.
Fuse's manifestation is fought in three stages: his first body, his second body and right arm, and his third body and left arm. Each body and arm spawn is formed with the defeat of the previous body. Defeating the final body counts as victory, even if the arms are left "alive." Fuse's body is rooted to the ground, and cannot move. The arms, however, are able to move. Fuse's body changes type each time it is defeated.
If the player moves within pistol range of Fuse, his corruption and eruption attacks have the potential to be triggered. His eruption attack, in particular, covers most of his throne and will instantly KO the player regardless of health. Staying at rifle range prevents their use, though Fuse can still use his body's abilities. Fuse's arms each possess the ability Fuse had in the previous phase of battle and can attack the player physically.
Fuse's body's attacks consist of a simple melee attack, a more powerful corruption attack, and an eruption attack which instantly knocks out anyone on his platform. Unlike other monsters in the game, Fuse does not heal when the player is knocked out, unless he has the heal power.
The player begins by fighting his main body, which has the stun ability and a quick melee attack. Once that is defeated, his right arm detaches with the ability to stun and his body gains the ability to heal; both parts have their own melee attack. A shattergun will allow both to be struck at once. Thanks to his healing ability, Fuse's body will likely take a significant amount of time to defeat. His final body causes his left arm to detach with the healing power and his body gains the damage power; Both have a melee attack as before. You will achieve victory after defeating the body a third time.