The array.

Fearing all would be lost, a group of Forerunners developed a plan to stop the parasite once and for all. They called this plan Halo. Using a massive installation known as the Ark, they built seven ring worlds, which they spread throughout the galaxy. This Halo Array, as they called it, when fired, would destroy all sentient beings within range, essentially starving the Flood to death.

Then, after staying the hand of judgment as long as they could, the Forerunners finally fired the rings and seemingly vanished from existence. The knowledge of the Halo Array and the Ark was lost for a time. All that remained was the hope that someone, somewhere in the distant future, would discover the truth.