Zasz was one of the most clever cerebrates, who never ceased to question his fellows in an irritating, sniveling manner. He coordinated the actions of multiple cerebrates and spied on enemies over great distances. While somewhat unstable, he was extremely loyal to the Overmind.
The Garm Brood was part of the zerg Swarm's main attack wing, along with the numerically far superior Jormungand and Surtur broods. The brood made up for its relatively small numbers by employing guerrilla tactics, including surprise attacks and frequent raids. It was known both for its extreme ferocity and the speed at which it attacks.
During the Great War, Zasz offered counsel to the new cerebrate created to keep safe the chrysalis containing Sarah Kerrigan. On Tarsonis, Zasz directed the young cerebrate to remove an immediate threat to the chrysalis by sweeping the area clear of nearby remnants of Omega Squadron.
Once the new cerebrate was strong enough to travel to Char the chrysalis was taken to a space platform and safely through protoss forces. Zasz warned the junior cerebrate its "first jump through warp space could be... unsettling."
The Terran Dominion's Alpha Squadron, led by General Edmund Duke, engaged first and was driven off. Zasz instructed the junior cerebrate to use the lull and relocate the chrysalis to a more secure location. The new location was then discovered and beset by Raynor's Raiders. As the battle raged Zasz urged the junior cerebrate not to allow anything to interfere with the chrysalis' imminent hatching.
Once hatched, Kerrigan desired to raid the Amerigo to obtain the means to break the ghost conditioning hampering her psionic abilities. Zasz was opposed to what it regarded as a display of personal aggrandizement, unsuitable for a servant of the Overmind. The Overmind assured the senior cerebrate Kerrigan's wishes were in accordance with its own and Zasz relented to assist. Nonetheless, Zasz and Kerrigan were to share only an adversarial relationship.
Freed of ghost conditioning, Kerrigan detected the presence of protoss on Char, and was subsequently challenged to single combat by Tassadar. Zasz detected something strange about Tassadar and counseled caution. The headstrong Kerrigan would have none of it and threatened Zasz, much to the latter's exasperation.
Zasz's advice was prescient; the duel turned out to be a distraction. While Kerrigan fought Tassadar's illusion, Dark Prelate Zeratul approached the cerebrate and claimed to know Kerrigan's weakness. The cerebrate was lulled into a false sense of security and ordered its brood away, allowing Zeratul to approach even closer. This was Zasz's undoing. The prelate revealed zerg were vulnerable to the Dark Templar's energies and slew the cerebrate.[note 1]
The death of Zasz came as a shock to the Swarm. While Kerrigan initially gloated at the cerebrate's demise, even she could not deny the implications of an attack which nullified cerebrate reincarnation.
For a time the Overmind was silent, but after the destruction of the Garm Brood it revealed that more than just the death of an immortal had taken place. When Zeratul's blade struck Zasz, the Overmind and the prelate were able to read the other's thoughts. Zeratul discovered the Overmind's origin, and the Overmind learned the secret location of Aiur.
- A different account of these events is presented in StarCraft: Queen of Blades. In this case, Zeratul killing Zasz was unrelated to Tassadar's actions and the timing was coincidental.