SUMMARY
Around fifteen million years ago, the Forerunner race was created by the Precursors in an effort to make a new species that could assume the Mantle of Responsibility. The Forerunners never lived up to the expectations of the Precursors, and they tried to remove all Forerunners from the galaxy, spurring the Forerunner-Precursor War.
RELATED MEDIA
These events are referenced in the novel Halo: Silentium, which was the subject of a Halo Book Club on January 31st, 2018.