Zamasu was a male Shinjin Jedi Knight who fell to the dark side of the Force and became a Dark Lord of the Sith, known as Darth Tyranus. Born into a noble family on the planet Kaishin, he was the heir to vast wealth and the noble title of Count. Zamasu was taken by the Jedi Order as a child and apprenticed to Jedi Master Gowasu.

Zamasu spent nearly fifteen years as a Jedi, but a disastrous battle on Galidraan shook his faith in the Order and the Galactic Republic. He fell under the influence of Supreme Chancellor Kann of Naboo, and left the Jedi Order. After learning that Kann was secretly the Sith Lord Darth Sidious, Zamasu turned to the dark side and became a Dark Lord of the Sith himself, replacing Darth Maul as Sidious's second apprentice. His new master bestowed upon him the Sith title of Darth Tyranus.

Reclaiming his birthright as Count of Kaishin and his vast fortune, Zamasu conspired with Sidious to force the galaxy into a war that would bring the Sith to power. He forged ties between corporations and planets discontented with the Republic, and became the leader of the Separatist movement that coalesced into the Confederacy of Independent Systems. Thousands of star systems flocked to Zamasu's banner and seceded from the Galactic Republic.

As the political leader of the Confederacy, Zamasu led the organization against the Republic in a conflict that would become known as the Clone Wars. The war culminated in a great battle over the Republic capital of Coruscant. In a duel aboard the Separatist flagship Invisible Hand, Jedi Knight Korn defeated Dooku. After being betrayed by his Master, Zamasu escaped kept hunting down Jedi from the shadows, including his former Jedi Master Gowasu, until he was confronted by Jedi Knights Cabba and Cus on the planet Coruscant, who he defeated with ease, even severing Cabba's left arm, Zamasu was forced to flee after Jedi Master Whis came and dueled Zamasu. Following this duel, Order 66 took place, as Sidious succeeded in destroying the Jedi at the end of the war, and he transformed the Republic into the Galactic Empire and proclaimed himself Emperor. Zamasu escaped from the planet and laid low.

During the Empire's reign, Zamasu dwelled on the ancient Sith world of Malachor after his ship crash-landed there in the hopes of unlocking the superweapon at the heart of the Sith temple. It was his hope that he could use the temple to destroy Sidious and his apprentice, Darth Vader—whom Zamasu viewed as the embodiment of everything he could have been. He also learned that his nemesis Cabba was still alive. Seeking to find hope and purpose, Zamasu sought out Cabba on the desert planet of Tatooine. Following a duel, Zamasu was ultimately killed by Cabba; ending his quest for vengeance, and finding a sense of peace in the knowledge that the end of the Sith was at hand.