History
Halo was built by the Forerunners as a giant weapon. This ring shaped world was the forerunners final option to stop the flood. It was fired and destroyed the flood and all life in the entire galaxy. After the galaxy had no more flood in it the machines the forerunners build replaced all life that was lost.
Halo was then discovered again in the year 2552 by the UNSC and Covenant who were already at war. The ring world to the Covenant forces was to start their great journey.