Dragon Ball Super: Season 4, Episode 15

4 - 15: Zamasu's Ambition The 'Human Elimination Plan' is Divulged

Goku Black explains that he is Zamasu from an alternate timeline where he succeeded in killing Gowasu, the Time-Ring allowing him to be unaffected by his past self's death. The other Zamasu then formally introduces himself as the Zamasu of Future Trunks' timeline, whom Black allied himself with as they destroyed the Super Dragon Balls after using them to grant Zamasu indestructibility. They then killed the other deities so they would not interfere in their goal of wiping all mortal life for their ideal utopia, saving Earth for last to have the humans suffer an agonizing demise. In the battle that follows, Zamasu and Black provoke Goku into giving the latter a challenge after revealing that Black also killed Chi-Chi and Goten when he acquired his new body and killed the alternate Goku. The two manage to defeat Goku before turning their attention to Future Trunks and Vegeta, both telling the former that his constant meddling in the timeline is the reason for Earth's misfortune. This infuriates Future Trunks, who explodes and transforms into an immensely powerful new Super Saiyan form.