Te Time
Te is a Gas Giant that is the homeworld of the Hunters. Initially a Forerunner outpost, it was abandoned by the idiots after they discovered the Hunters living on chunks of rock in the planet. The Forerunners built many gas mines all over the planet and some were initially hit by these chunks of rock and overwhelmed by Lekgolo Worms, which, when provoked, would combine to form Mgalekgolo (aka Hunters). They drove the Forerunners off the planet and began slowly digesting their facilities.
Nearly 100,000 years later the Covenant arrived on Te and discovered that dozens of Forerunner Mines had been destroyed by the Hunters. Seeing this as Heresey, they declared war on the Hunters but could not attack them easily as their ships could not get close enough to the Gas Giant without being pummeled by rocks and EMP.
The planet was abandoned by the Covenant who took the 'nice' Hunters with them, who lived on Forerunner mines instead of consuming them. This left the 'not nice' Hunters to devour the entire solid mass of the planet. However, due to their incredibly slow digestive methods, it is predicted that this will take them a further 200,000 years.