From Halopedia, the Halo wiki
The Draetheus system is a stellar system consisting of at least five planetary bodies. The system is home to Draetheus V, a former Forerunner colony, and later a human research colony established after the Human-Covenant War.
In 2554, Covenant forces lead by Sangheili Zealot Merg Vol attacked the planet for the Forerunner relics present there. Eventually the Covenant forces discovered that Draetheus V's moon, X50, was actually a Forerunner device capable of building and destroying planets. The Covenant used the device to disintegrate Draetheus V, however Spartan Sarah Palmer killed Vol and sabotaged the device, stopping the dismantling process of the planet.
List of appearances