From Halopedia, the Halo wiki
The planet was the site of a battle during the Forerunner-Flood war. At the beginning of the engagement, 12,423 small recreational vessels and seven freighters, all controlled by the Flood, appeared within the planets orbital perimeter. The Flood used the smaller ships as an ablative armor, taking all damage and protecting the freighters, which descended to the planet's surface and unloaded its passengers over large population centers.
Although the local Forerunner orbital forces were prepared for an invasion by the Flood, they were unable to prevent the infection of the planet and were overrun in a matter of days. Rather than allow the Flood to consume the planets civilian population, the Forerunner Navy destroyed its own cities from orbit denying them to the parasite for use as biomass.
List of appearances