Dyson sphere
From Halopedia, the Halo wiki
The Micro Dyson Sphere is located in the center of Onyx, protected by a slipspace bubble. It is one hundred fifty million kilometers or two stronomical units, the equivalent to the distance of the Earth orbiting its sun. The Micro Dyson Sphere has a spectra and energy output slightly smaller than the dimensions of Sol. This was going to be the shealther for the Forerunner to go, when Halo activated but no one knows what went wrong.