As being fascinated with, if something is wary of, the dark templar, Tassadar is a leader of the galactic fleet that created first contact with earthly beings known as Terrans by eliminating their colony of Chau Sara to have infestation by Zerg. But he disregards his orders to continue destroying various worlds without concerns for the Terrans and instead engages the battle with Zerg by conventional means. After tracking down the Zerg to their planet Char, he discovered the dark templar named Zeratul and the human Jim Raynor, being friendly to those two and learning how to manipulate psionic powers in conjunction with the dark powers, which was came from the dark templar.
The Khalai military encounters Tassadar's contortion with the dark templar Zeratul as heretical and as a bigger threat to their society than the Zerg occupation of planet Aiur, the home world of Protoss, sending both Aldaris and Artanis to apprehend him. Artanis however, along with the fleet commander Fenix, sides with Tassadar himself, starting a horrible civil war between his followers and the military government. The conflict is only ended up with his demonstrations of the effectiveness of the dark templar against the Zerg invaders. With the cooperation of Jim, Zeratul and Fenix, Tassadar breaks through the offensive attack against the Zerg defenses on Aiur to their biological mastermind called the Overmind. He channels both his psionic energy manipulation and those of dark templar through the hull of his flagship named Gantrithor and crashes it into the Overmind. At the end, the resulting discharge of energy defeats the Overmind and transcends Tassadar to the latest level of spiritual existence in order to transform into a twilight messiah by Metzen's references.
Following the events of this game, Tassadar is now dead by the Protoss race, though he became a legendary figure among them. However, when Zeratul went to the remnants of the Zerg Overmind in his investigations into the future of Koprulu Sector, Tassadar appears to be a ghost and shows him a vision a vision that the Overmind had first shown him if he destroyed it. This vision foretold the fall of the universe to the Zerg led by the fallen Xel'Naga named Amon and his Protoss-Zerg hybrid minions. This counterpart of Tassadar turns to be a projection of the Xel'Naga Ouros and not the real one.