The Forerunners were a race of highly technologically advanced humanoids that died at over 100,000 years ago, and constructed the Halo Array. They fought the Flood, and after a 300-year conflict, they fired the Halo rings as a last resort, killing the Flood, themselves,a and every other sentient lifeform in the galaxy. But before that, they cataloged every species in the galaxy, and protected these embryos, and even living specimens. They then reseeded life on their homeworlds through out the Milky Way. But they themselves are presumed extinct.