The Forerunners are a supposedly extinct species. They are a huge mystery in the Halo universe, but we have learned more about their story through the terminals in Halo 3, Halo: Combat Evolved Anniversary and Halo 4.
The Forerunners were an advanced civilization with a big problem, the Flood. The Flood were a major threat to both their civilization and their beliefs. They believed in diversity in the universe, and the Flood converting their species was not what they wanted. The Forerunners created the Halos, galactic weapons of mass destruction, in order to kill the Flood. Originally, they had hoped to survive the lighting of the Halos by escaping to shielded locations, such as the Ark. This plan was foiled though when their own advanced AI, Mendicant Bias, ruined their plans. Mendicant Bias was manipulated by Gravemind and planned to deactivate the Halos, allowing the Flood to survive. The only way he could do this was by accessing the Ark. To access the Ark, he needed a special Forerunner ship called a keyship. The keyships allowed the Forerunners to enter shielded locations, such as the Ark. They had to destroy them just to make sure Mendicant Bias didn't deactivate the Halos. They also created another AI, Offensive Bias, who was designed to slow down Mendicant Bias. In the end, the Didact was forced to light the Halos unshielded, making the ultimate suicidal sacrifice for the galaxy over 100,000 years ago.
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