Zamasu is a Kai from Universe 10. He is a fighting prodigy and pupil of the Supreme Kai Gowasu who tutors him as a replacement. While studying the purpose of the Kais as creators, Zamasu began to harbor resentment for mortals due to their behavior and eventually the Kais themselves for creating such imperfection.
Zamasu first appeared in the Dragon Ball Super anime episode "Master and Student Reunited - Son Gohan and "Future" Trunks." His first manga appearance was published in V Jump 11/2016 in Dragon Ball Super Chapter 16.
Major Story Arcs
While Zamasu continued his instruction he learned of the Super Dragon Balls as well as the Rings of Time. After seeing Goku, a mere mortal, surpass the power of the Gods, Zamasu finally made plans to kill his teacher and begin his evil plan to destroy the mortals of the universe.
As he was going to kill Gowasu, he was stopped by the God of Destruction Lord Beerus along with Whis and Goku. They had become suspicious of him through knowledge of future events. Zamasu was easily destoyed by Beerus who assured everyone that by killing Zamasu any future versions of him would be destroyed as well.
Due to the nature of the Rings of Time, this turned out to not be true. Zamasu killed Gowasu and took his Potara to become the Supreme Kai. With his knowledge, Zamasu obtained the Super Dragon Balls and wished to switch bodies with Goku. In Goku's powerful body, he went to Earth and killed Goku and his family and began his 'Zero Mortals Plan.' The people of Earth would call him Goku Black.
In order to begin his plan, he needed a reality with no gods to stop him. He found the Universe 7 future timeline of Trunks where the Supreme Kai had been killed in battle with Babidi and Dabura. When the Supreme Kai passed, so did the god of destruction, which left none powerful enough to stop him. In his new timeline, he went to Universe 10 and killed Gowasu and formed an alliance with his future self. They shared the same views and continued the plan together.
They found the Super Dragon Balls and wished Zamasu immortality. They would find them again in a years time and wish for their destruction.
Trunks went back in time to receive aid from his mother as well as Goku and Vegeta. Goku, Vegeta and Trunks were defeated several times by Zamasu and Black but they always escaped and returned. When Trunks almost defeated Zamasu by sealing him with the Mafuba attack, Zamasu realized that the mortals are not as weak as they believed. Using their Potara, Zamasu and Black fused to become one incredible powerful being.
Goku and Vegeta were no match for the fused Zamasu so they fused into Vegito to combat him. Since Zamasu's immortal body was now fused with that of a mortal there were some side effects and mutations as well as his body not being immortal. Their fusion was cut short though and Zamasu once again had the upper hand. Trunks was able to take the energy of the people of Earth (as well as Goku and Vegeta) into his sword and slice Zamasu in two.
But since Zamasu was immortal in spirit, he was able to escape death and infect the atmosphere of the Earth. With his enemies unable to harm him, Zamasu used energy attacks to kill the last remaining survivors of Earth. Out of ideas, Goku used his communication device to summon the Omni-King. When Omni-King arrived, he was displeased with the horrible place Zamasu had created and the heroes were able to escape just in time before Omni-King destroyed Zamasu along with the entire reality.