Aiur is the homeworld of the protoss race.
During the Great War, when Zeratul killed cerebrate Zasz, the Overmind managed to connect with his mind and discover Aiur's location. The Overmind took most of the swarm to the planet and killed off 70% of the population. Tassadar merged his psionic energy with Void energy and crashed his ship into the Overmind to save the remaining protoss.
At one point in 2504, Zeratul returned to Aiur to seek out the Overmind's tendrils, believing them to contain an ancient prophecy. While there, he reactivated many structures, freeing any protoss within. The zerg tried to attack Zeratul's base, defending the Overmind's corpse, which, at the time, served as a food source. Zeratul managed to retrieve the psychic fragments within the Overmind's tendrils, and was greeted by Tassadar's spirit, who told him of the coming darkness.
In 2506, Artanis took the Khalai and a few Nerazim to Aiur to reclaim it, despite Zeratul's warning. Amon took control of most of the Khalai, while Zeratul freed a few others, including Artanis and Karax, but at the cost of his life.
After receiving reinforcement from the Purifiers, Tal'darim and Nerazim, the unified protoss returned to Aiur and disabled the psi-matrix to delay the corrupted Golden Armada's return. The Daelaam managed to destroy Amon's host body and use the Keystone to free the remaining Templar. After the battle, the remaining protoss rebuilt their cities on Aiur.
Alarak declined an alliance with the Daelaam and set off to create a new Tal'darim homeworld, but he offered his people one chance to join the Templar.