Forerunners are an old civilization that was the dominant race in the Milky Way for around 10,000,000 years. They upheld the Mantle of Responsibility, which is guardianship of all life in the galaxy. Approximately 100,000 years ago, the Flood entered our galaxy from beyond its borders and laid siege to the Forerunners. Misappropriating the Flood as a disease rather than the true cosmic horror it is lead the Forerunners to severely misjudge the Flood, allowing them to make serious gains into their civilization. By the time the Forerunners realized the true threat the Flood had posed it was too late. Eventually, they made use of Halo, a weapon initially designed to combat the Flood's advance. A weapon capable of killing sentient life. Tuned to a proper frequency, Halo could encompass the entire galaxy. Which is ultimately what was done in the end. The surviving Forerunners, realizing that they had failed the Mantle, left the galaxy, and placed humanity as their heirs to the Mantle.