Shield world

The Shield world is a Forerunner construct built to shield them from the blast of the halo rings going off. They also seem to be related to Cortana's lines from the Halo 3 Announcement Trailer:

Cortana: (distorted) "...I have defied gods and demons..."
Cortana: (distorted) "...I am your shield. I am your sword..."
Cortana: "I know you. Your past, your future..."
Cortana: (voice over) "This is the way the world ends."

They work by "shielding" the Forerunners (or possibly Reclaimers) via a Micro Dyson Sphere inside a Slipspace rift. This makes its a proverbial Forerunner 'bomb shelter' to the activation of the Halo installations. While there could be others, only one, in the centre of the planet Onyx, has been revealed.