The complex history of Goku Black’s appearance has finally been revealed in the latest Dragon Ball Super episodes. It is revealed that Goku Black is actually Zamasu, former North Kia of Universe 10, who used the power of the Super Dragon Balls to swap bodies with Goku. Once Zamasu possessed Goku’s body he traveled to Earth and killed Goku and his family. Goku Black’s ultimate plan is to destroy every mortal across all times and universes.
To make that plan a reality Zamasu had to find a Universes without a God of Destruction since Kai’s can’t interfere with planets. After searching he eventually found Future Trunks’ universe which didn’t have a God of Destruction, and teamed up with himself (future Zamasu) and once again gathered the Super Dragon Balls this time wishing immortality for future Zamasu, and then after waiting a year wished for the Super Dragon Balls to be destroyed. This eventually leads us to events at the beginning of the Super Trunks Saga.