Future Zamasu's History
Background
Future Zamasu existed in Future Trunks' timeline, he was once a Supreme Kai apprentice training under the Supreme Kai, Future Gowasu. When Goku Black initially meets Future Zamasu, he implies that Future Zamasu had also wanted to become a Supreme Kai and kill Future Gowasu, as Black states after he meets Future Zamasu, "You also wanted to do this, right?", in reference to Black killing Future Gowasu just prior to meeting Future Zamasu.
One day, when serving Future Gowasu tea, he found his master killed under the hands of a mysterious man. The stranger then reveals himself to be Zamasu, from another timeline. He had come here looking for a timeline and universe without a God of Destruction, as Supreme Kais are not allowed to lay a finger on the planets. His search through all realities had eventually brought him to Future Zamasu's world where Universe 7's Destroyer, Future Beerus was deceased. Zamasu offers Future Zamasu a new opportunity to begin the "justice" the world deserves, in which Future Zamasu agrees to. The two then gathered the Super Dragon Balls and wished for immortality for Future Zamasu then destroying the Super Dragon Balls so no one could revert their wishes.
With no one standing in their way, Zamasu and Future Zamasu then initiated their plan: to kill "sinful mortals" and "unnecessary gods" in order to bring "justice" and to create a "utopia" in the future timeline, as well as all of existence. After killing all of the Supreme Kais, and thus by extension the Gods of Destruction, Zamasu unleashed his wrath on countless planets, killing as many mortals as possible while Future Zamasu stood back, watching their plan unfold.
"Future" Trunks Saga
Only first trip to the future, as Goku and Goku Black began engaging in a heated battle, Zamasu appeared next to Black, reminding Black of his promise to allow him to kill Goku. He then attacks Goku at the same time with Black, forcing Trunks to jump in and even up the fight. During the fight, Trunks is fighting evenly with him as a Super Saiyan 2, but when Trunks stabs him in the stomach with his sword, he instantly heals his wound. Zamasu also emerges unscratched from Future Trunks' Final Flash, revealing his body is immortal. Goku regroups with Future Trunks as Goku Black charges a Super Black Kamehameha, with Zamasu grabbing them and holding them in place to prevent them from escaping. Goku and Trunks are defeated by the attack, and Zamasu remained unharmed due to his immortality. With Black, Zamasu prepares to finish off the two Saiyans, but they are interrupted by Vegeta, who woke up and intercepted their attack but fell back unconscious immediately after. In the confusion of the attack, Goku and Trunks have disappeared, so Future Zamasu and Black prepared to finish off Vegeta, but they are forced to fall back when members of the Resistance fire tear gas at them. The two then spot the time machine with the beaten Trunks, Vegeta, and Goku inside, and try to intercept it, but are too late. Zamasu exclaims they will return eventually, and when they do they will meet their end.
Both Future Zamasu and Black are next seen at the cabin they are living in, patiently waiting for Goku and his friends to return. Goku Black and Future Zamasu discuss about killing Goku and how they aren't strong enough as opponents. Future Zamasu says if Goku Black kills Goku too quickly, he'd lose an opportunity to grow stronger by adapting to his fighting style, suggesting that he kills Goku slowly and carefully. He then offers Black the chance to become immortal, but Black refuses, saying he is satisfied with the power he has now.
In the second trip, Future Zamasu and Black soon sensed the trio had returned and followed suit on their location. They then met Goku, Vegeta, and Trunks and immediately started trading blows once more. Goku then tells them that they knew their plan, in which Future Zamasu was a bit shocked, but delighted asking them if they really knew it. He then tells him the full plan, in which Black replies is incorrect, claiming to be another version of Zamasu in Goku's body.
While Future Zamasu and Goku Black fight and constantly defeat the Saiyans, they tell the true story of their past, how Goku Black assumed Goku's body, killed him along with Chi-Chi and Goten, and traveled to the future and killed Gowasu, causing Future Zamasu to reveal his true nature to him and become partners with him, and how he used the Super Dragon Balls to become immortal, then destroying them. Future Zamasu is knocked back by an angry Goku upon telling this story, with Present Zamasu soon defeating him. They later mock Trunks, as he was one of the reasons they did this in the first place which enraged the latter and tries to attack on his own. Black managed to grab Trunks and fires a blast through his stomach, knocking him down, while Future Zamasu stops Vegeta from interfering.
They later notice Future Trunks assume a never-before-seen Super Saiyan form after explaining their "Zero Mortals Plan". Future Trunks with this form fights with Black, with Future Zamasu and Future Yajirobe surprised at how much he had improved with the form. The two seemed evenly matched, which prompted Future Zamasu to step in help Black against Trunks. However, every attack they try, even the combined Energy Ball, were endured and countered by Trunks in his form. Trunks holds the two off while the others escape in the Time Machine. Black replies that it is okay, since they will come back later and resumes the fight with Trunks.
When the fighting resumes in the forest outside of the cabin Black and Zamasu have been staying at, Future Trunks reveals that he has found a weakness with Zamasu and Black's strategy, by throwing a powerful attack at Black Zamasu would throw himself in front of it to block it and would be forced to regenerate himself due to the damage thus leaving Black to be actually vulnerable to the real attack. Using this new strategy Trunks was in fact able to momentarily knock out and defeat Goku Black. However, before Trunks could attempt to destroy Future Zamasu's body, Black re-enters the battle and impales Trunks through the spine.
Soon after, Black and Future Zamasu notice the Time Machine back in their timeline, indicating that Goku, Vegeta and Bulma have returned once more. They approach to the group, destroying the time machine, proclaiming that this timeline will be their grave, only for Vegeta to correct their claim, telling them that it would be theirs. However, Vegeta and Goku zone out for a little bit, arguing that their "grave" is destroyed and actually don't know what to do. They then resume their conversation and begin the fight. With Goku deciding to fight Future Zamasu once more. However, they are interrupted by Black's former master and Universe 7's Supreme Kai, Shin, from his timeline. Gowasu berates on their actions, calling them "horrible" and "immoral". Black responds to the question that they are talking about, telling them they believe that they are doing all of their actions for a greater cause. Future Zamasu and Black proceed to try to kill the Supreme Kais but are saved by Goku and Vegeta. Goku and Future Zamasu soon continue their fight, with Goku overwhelming Future Zamasu blow by blow, but to no avail, due to his immortality.
After Black created multiple regenerative clones through a slash from his anger-fueled energy scythe, Zamasu began heading towards where Trunks, Mai and Bulma were. As this was going on, Goku's usage of Instant Transmission was blocked through the rift, preventing him and Vegeta from chasing after Zamasu. After Zamasu reached his destination, he is soon confronted by Bulma, who tries to flirt with him to buy her son enough time to master the Evil Containment Wave, but to no avail, as Zamasu angrily grabbed her through the collar and knocked her out. Upon seeing this, Trunks is quickly reminded of Future Bulma's death and successfully unleashed the Mafuba on Zamasu, placing him in the jar. Bulma soon regains consciousness and orders Mai to seal the jar with the talisman, but to their shock, the talisman was forgotten at Master Roshi's house. The three of them tried to hold the jar to prevent Zamasu from escaping, but to no avail as Zamasu escapes barely breathing.
Upon sensing that Zamasu's ki is getting weaker, Black uses Instant Transmission, teleporting himself to Zamasu. Black asks Zamasu what happened, with the latter warning Black that mortals mustn't be underestimated and that their plan could be crushed if they let their guard down. After finding that the time is ripe, Zamasu put his Potara earring on his right ear, and the two of them fused together to create Fused Zamasu. Due to Black being a mortal, the fused Zamasu is mutated since it is a fusion of an immortal and a mortal. After fighting Goku, Vegeta, Vegito, being sliced in half by Trunks, and finally being erased from existence along with the universe, by Future Zeno, the fused Zamasu is defeated, and ultimately, Future Zamasu is too.
After this, Whis goes to the future before Future Beerus' death and preemptively warns him about Future Zamasu, creating a new timeline free of Future Zamasu's tyranny.
