Future Zamasu was the Future Trunks timeline counterpart to the original Zamasu. He teamed up with his original counterpart, Goku Black, to destroy the mortals as part of the original counterparts plans. Unlike Black, this version of Zamasu had immortality.
Like Goku Black, Future Zamasu was as sadistic as he was and had the same level of hatred for mortals. When he saw that his master was murdered by Goku Black, Zamasu was at first shocked but soon embraced his dark side and joined his original counterpart to complete the plan. Unlike Black who was prideful because of his powerful body, Future Zamasu was prideful because of his immortality. He believed that he was invincible in battle and that no one could defeat him.
Like Present Zamasu, Future Zamasu has the same powers and abilities. Unlike his original counterpart, Zamasu has a higher stamina and can resist attacks that can destroy him. Like the other incarnations, Future Zamasu can fly, has superhuman strength, and can fire laser beams from all ranges. He was able to fight Goku and Vegeta in battle on their level.
Despite his immortality, Future Zamasu can still be sealed away. When he confronted Future Trunks, he ended up getting sealed away.