Fusion Zamasu's History
"Future" Trunks Saga
Future Trunks uses the Evil Containment Wave on Future Zamasu after Bulma distracts him, to seal him away in a vase, with Future Mai closing it with the lid. Unfortunately, Goku forgot to bring the talisman made by Master Roshi, allowing Zamasu to break free. However, he is left weakened by the effort of escaping and was left deeply shocked that his enemies had discovered a means of actually neutralizing him despite his immortality. When Black arrived, having sensed Zamasu's ki decreasing, the latter informs his counterpart that they could no longer afford to underestimate the mortals.
Declaring that they would display the true power of gods, Zamasu and Black stand beside each other in the sky, with the former taking his Potara earring from his left ear and attaching it to his right ear. This initiates an explosive Potara Fusion between the two villains, creating Fused Zamasu. Upon his birth, he declares his new form to be the embodiment of justice and the world. He orders everyone to revere and praise him, calling himself "The Supreme God".
As he manifests a halo of light, Fused Zamasu proclaims his body's abilities, before attacking the Saiyan duo with lightning. Vowing to show them the power of a god, Fused Zamasu launches exploding blades at Goku and Vegeta. After the pair recovers, Zamasu tells them the Project Zero Mortals will be completed, follows it by attacking with lightning again, and withstanding Vegeta's attack. Deciding to unleash his godly might, Zamasu emits a bird-like ki figure, which repels the Saiyan's, damaging them to such a degree that they revert to their base forms. Overlooking the defeated pair, Zamasu directs his attention to Earth and unleashes his power in an attempt to clean the planet of its filth. However, Vegeta and Goku recover and attack, destroying Zamasu's bird-like ki technique in the process. When they attack Zamasu himself, he blocks their attack and follows it by attacking with his exploding blades again.
When Future Trunks arrives, the two briefly fight, leading to Trunks being repelled to the ground. Insulting the mortal, Zamasu attacks with Holy Wrath, which clashes with Trunks' Galick Gun. As Zamasu's attack overpowers Trunks', Vegeta joins his son and the two launch a combined Galick Gun, which pushes Zamasu's attack back to him, but causes little to no damage at all. Frustrated at their resistance, Zamasu attempts to attack Trunks but attacks Vegeta instead when he stepped in front of the attack. Afterward, Goku approaches Zamasu and fires his God Kamehameha, which clashes with Zamasu's Holy Wrath.
Zamasu, however, is overwhelmed by Goku's attack, resulting in an explosion which surprises the semi-god, leaving the right half of his face and body swollen and turning purple. Goku then attacks Zamasu at close range, but Zamasu grabs him by the leg; Goku then enhances his physical attack by combining his Super Saiyan Blue form with Kaio-Ken, and kicks him to the ground, managing to destroy the halo in the process. Goku ends up too exhausted to attack further and collapses afterward. Zamasu then screams Goku's name in anger and hate and charges up by calling upon the Light of Divine Justice to spread his mutated arm.
As Vegeta and Goku manage to fuse into Vegito, Zamasu wonders to himself why mortals try and imitate the gods, declaring it sinful. He is interrupted by Vegito's punch square in the face because of getting tired of his talk. Vegito ascends into Vegito Blue, and the two begin their battle, and at first, Vegito seems to have the upper hand. Vegito mocks Zamasu, asking if this is really the power of a god. Zamasu angrily knocks Vegito to the ground. Zamasu then tried to finish off his opponent with his ki blade, but Vegito counters with one of his own. After Vegito explains that Zamasu is technically also mortal due to fusing with Goku's body, Zamasu stated that's what he wants; by obtaining the power of Goku, who surpassed the gods despite being mortal, he symbolized all the failures the gods have made, atoned for the sins of mortals and the mistakes of the gods.
Zamasu cries (much to Vegito's disbelief), saying that his tears will purify the world, before powering up, resulting in his right side growing more grotesque. As the two resumed their fight, Vegito taunts Zamasu for not being able to keep up with his speed and fires his Final Kamehameha, but Zamasu emerges unscathed. As Vegito uses up all his power to sustain his fusion into his Final Kamehameha, the fusion time runs out and Vegito defuses back into Goku and Vegeta, leading to Zamasu attacking the pair and defeating them effortlessly. Before he could kill the two, Trunks arrives and blocks Zamasu's attack.
Fused Zamasu, whose mind has completely succumbed to hatred and insanity, then fights Trunks, while accusing the mortal of being weak, and mockingly asks where he will seek help from next, but Trunks responds by saying that he is not ashamed of his "mortal weakness". As Trunks held out against Zamasu, he unconsciously gathered ki from Earth's inhabitants. Goku and Vegeta contribute their ki too, and with everyone's power channelled into his sword, Trunks charges at Zamasu. Unable to defend himself from Trunks' enhanced sword technique, Zamasu is stabbed. While in disbelief of mortals retaliating against him and his "justice", and also asking Trunks if he'd dared to go against the "justice" of a god, Trunks declares his "justice" meaningless, before slicing Fused Zamasu in half, destroying his body in the process. However, Fused Zamasu's will lived on, only to be erased later by Future Zeno.
