Shield world

Shield World is the translated Forerunner term for a miniature Slispace-based Dyson Sphere. The Shield World is a massive sphere in Slipspace, featuring an Earth-like terrain and atmosphere on the inside of the sphere with a small star in the center.

Spoiler warning: Halo: Ghosts of Onyx plot and ending details follow.

The Shield World was constructed by the Forerunners at the center of the artificial planet Onyx. It is encased in a Slipspace bubble so that, though it is well over a hundred million miles in diameter, it only occupies a few square meters in the "real" dimension. The tiny, meters-wide Core Room Antechamber that exists in real, non-Slipsace was protected by billions, perhaps trillions of Onyx Sentinels that formed the "mantle" of the planet Onyx (which was covered by a terrestrial crust, giving Onyx an Earth-like surface).

In Halo: Ghosts of Onyx, Dr. Catherine Halsey theorizes that the Shield World was designed as a kind of "bomb shelter" for the Forerunners should the Halos be activated. In the Slipspace bubble, they would (presumably) be protected from the life-destroying effects of the Halo pulse.

In 2552, after Installation 05 sent a signal throughout the galaxy to put the remaining Halos on standby mode, Onyx's automated defense system was inadvertently activated. After a complex series of events and The Battle of Onyx, Dr. Halsey, SCPO Mendez, and a number of Spartan-IIs and Spartan-IIIs escaped to Shield World, and the core room antechamber was destroyed. So far they have encountered no one on the Shield World, leaving them to wonder whether any of the Forerunners managed to escape to the Shield World before the Halos were last activated a hundred thousand years ago.

Trivia

 * Onyx was referred to as "the Shield" of the Forerunner, while the Halos were "the sword." This echoes Cortana's line on the Halo 3 Announcement Trailer, "I am your shield, I am your sword," but so far Bungie has said nothing about a correlation between Cortana's use of the sword "shield" and Onyx's "Shield World."