Shield world

Shield World is the translated Forerunner term for a 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 star at 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 compressed within a Slipspace bubble so that, though it is over a hundred and fifty million miles in diameter, it occupies only a few square meters in "real" space. The tiny, meters-wide Core Room Antechamber, which serves as the portal to the Shield World and exists in real space, 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, when Installation 05 sent a signal throughout the galaxy that put the remaining Halos on standby mode, Onyx's automated defense system was also activated. After the events of The Battle of Onyx, Dr. Halsey, SCPO Mendez, and a number of Spartan-IIs and Spartan-IIIs escaped to the Shield World, while the core-room antechamber was destroyed. At the end of the novel, they have not encountered anyone on the Shield World, leaving them to wonder whether any of the Forerunners made it to the Shield World before the Halos were last activated.

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 word "shield" and Onyx's "Shield World."