Zamasu is the Supreme Kai apprentice of Universe 10. Zamasu is noted to have similar ki to "Goku Black", and is in someway connected to him.
He is killed by Beerus in the altered flow of time, however, the present Zamasu was still alive, despite the flow of time having changed as the result of Future Trunks going back to the past. Zamasu in the original flow of time succeeded in carrying out his plans to kill Gowasu and take his Potara Earrings and Time Ring. Following this, he used these items to gather and use the Super Dragon Balls to switch his body with Goku, becoming Goku Black, and traveled to Future Trunks' timeline and allied with his counterpart there. As Goku Black, the Time Ring he wore stopped him from being erased from existence once his past self was "killed", after time had been altered.
In the future, he killed Gowasu again and is an ally of the Zamasu of this time wich wished for immortality.