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 zerg 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.
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."
On Char the chrysalis' psionic emanations drew terrans and protoss and the Swarm was forced to defend their prize. Zasz warned the Overmind of the unwanted arrivals.
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 counselled 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,DarkPrelateZeratul 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.
↑Zasz: "Though you be the favored servant of the Overmind, you would do well to remember that you are just a servant. You know of our grand mission, Kerrigan. Would you put your personal whims before the will of the Overmind?" Kerrigan: "Do not cross me, Zasz. I will do as I see fit, and not you or any other Cerebrate shall stand in my way." Overmind: "Let her go, Zasz. The greatness of her spirit has been left to her; that the Swarms might benefit from her fierce example. Fear not her designs, for she is bound to me as intimately as any Cerebrate. Truly, no Zerg can stray from my will, for all that you are lies wholly within me. Kerrigan is free to do as she desires." StarCraft. Vivendi Games. Mission: The Amerigo (in English). 1998.
↑Kerrigan: "For the last time Zasz, you question my motives and authority at your own peril." Zasz: "You dare threaten a Cerebrate? You will be the doom of us all!" StarCraft. Vivendi Games. Mission: The Dark Templar (in English). 1998.
↑Tassadar: "Zeratul, perhaps the time has come to tell our friends of the foe we face." Zeratul: "Indeed. When I slew the Cerebrate on Char, I touched briefly with the essence of the Overmind. In that instant, my mind was filled with its thoughts, and I tell you now our worst fears have come true. The Zerg were indeed created by the ancient Xel'Naga, the same beings that empowered us in our infancy. But the Overmind grew beyond their constraints, and has at last come to finish the experiments they began so long ago." StarCraft. Vivendi Games. Mission: Shadow Hunters (in English). 1998.
↑Overmind: "Yet shall their overweening pride be their downfall. For when the assassin Zeratul murdered Zasz, his mind touched with mine, and all his secrets were made known to me. I have taken from his mind the secret location of Aiur, the protoss Homeworld." StarCraft. Vivendi Games. Mission: Eye for an Eye (in English). 1998.