Sentinels (Halo) - Forerunner

Forerunner

The Forerunners are a long-lost species which are first cryptically mentioned in Halo: Combat Evolved. Little is revealed about the Forerunners in the games, but some (notably, the Librarian) consider humanity to be most eligible as reclaimers of the Mantle; humanity is thus the only race which is allowed to retrieve the indices with which to activate the Halo network. According to the official backstory to the series, the Forerunners came to power after they successfully rebelled against their creators - the Precursors. To ensure that the Precursors will never again threaten their existence, the Forerunners exterminated them, save for one imprisoned. At the height of their power they formed a galaxy wide empire more than 100,000 years before the events of the main Halo story arc. The Forerunners successfully usurped the Mantle from the Precursors - protection of all life in the galaxy. Much later, they were threatened by the parasitic Flood. The Forerunners took action, but the parasite was unstoppable. A group of Forerunners conceived a plan to stop the Flood once and for all, building an installation known as the Ark that created seven ring-shaped megastructures called "Halos". The Halo Array, when activated, would destroy all sentient life within range—depriving the Flood of its food. After waiting as long as they could, and travelling the galaxy to collect species from planets, which would later be used to "re-seed" the galaxy, the Forerunners activated the Array and vanished. Although the parasite's spread across the galaxy was halted, the Forerunners paid a terrible price as they, and all sentient life capable of sustaining the Flood in the galaxy, were annihilated. The Covenant worship the Forerunners as deities and relentlessly search for Forerunner relics. The reason for the Forerunner's disappearance is revealed in Halo: Combat Evolved, when the artificial intelligence Cortana enters one of the Halo's computer networks and learns the true purpose of the Halos.