About me: Once the Supreme Kai of universe ten Zamasu had used the super dragon balls to switch bodies with Goku thus becoming Goku Black as the mortals would come to call him. Not long after unlocking super saiyan rose Black was approached by a very strange man who claimed to be a demon god from the future by the name of Fun. Fu told him that his plans would fail, he explained in great detail how Black would be pressured into fusing with Zamasu and how that fusion would be unstable and that Trunks would destroy their physical body causing them to merge with space and time until Goku would summon Zeno himself and Zamasu would be erased. Fu offered Black a chance to become stronger than he ever imagined and quiet possibly complete his zero mortals plan. Black was intrigued by this offer and asked how exactly he would go about achieving all of that and in response Fu offered him a crimson time breaker mask and told him that if he traversed other timelines and killed Goku over and over again he would master his new body and become stronger even than the fusion of Black and Zamasu that was erased.
Black accepted Fu's offer and hunted down 99 different versions of Goku learning from each battle. By the 86th Goku he was introduced to Goku's Ultra Instinct Omen and killed it. But when Black met the 100th Goku he knew this one had a power beyond Ultra Instinct Omen and he pushed Goku as hard as he could but Goku retreated because he couldn't defeat Black and so he trained in some chamber of spirit and time or something very similar. When next they met Black had evolved his ssjr to ssjr3 but Goku confronted him in the perfected Ultra Instinct just as Black had wanted.
Unfortunately Goku could hold the perfected Ultra Instinct for long and Black had evolved even further to full power ssjr3. Even Vegeta in his new ssjbe controlled berserk form was no match for Black either and was easily swatted away like an annoying gnat but the gnat had many allies that were able to distract Black long enough for Goku and Vegeta to fuse using a technique Black had never seen before. Even the ssjb fusion of Goku and Vegeta wasn't enough to defeat Black but once again in his arrogance Black underestimated his mortal opponent and Gogeta took it to the next level with ssjbe and in one punch he broke Black's mask and seemingly killed him.
Unfortunately for the mortals Goku Black did not perish that day in his battle against Gogeta. True his mask was broken and he did disappear but he wasn't killed, he was simply pulled back to the timeline he left when he started his killing spree against Goku. And while he was heavily wounded there was no one left in this timeline who could possibly pose a threat to him. All the gods were dead, their angels inactive and Zeno none the wiser. Black was furious that he had been defeated by mortals yet again and he vowed to train harder than ever and get his revenge. If Goku could obtain Ultra Instinct then he should be able to do it to especially since he had fought against the omen form multiple times and even killed Goku while he was using that form multiple times. So he already had a decent understanding of at least the omen form he just needed to train and push himself to his limits but he needed a training partner that could actually withstand his power and spar with him. Black decided to use a time ring to find Xeno Merged Zamasu to become his training partner, while Zamasu wasn't as strong as Black he was immortal which meant Black didn't need to hold back and because he still had Saiyan in him he would also slowly grow stronger from Black's assault until he could withstand it and eventually fight back. Together they would become stronger than ever and unlock a deeper power within themselves and then project zero mortals could finally be achieved.
