Shield world

Shield World is the name for the micro Dyson Sphere constructed by the Forerunners around the artificial planet Onyx. Shield World was a massive sphere in space, the inside of which contained habitable ground and atmosphere, with a star in the middle. Shield World was encased in a Slipspace bubble so that, though it would likely have been well over a hundred million miles in diameter, it only occupied a few meters length in the "real" dimension.

Shield World was designed as the escape for the Forerunners should The Halos be activated. Shield World had a complex system for defense, and the only known access point was from the Core Room Antechamber in the center of the planet.

In 2552, after Installation 05 sent a wave 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. Catherine Halsey, SCPO Mendez, and a number of Spartan-IIs and Spartan-IIIs escaped to Shield World, and the Core room antechamber was destroyed. However, they found that, for some reason, no Forerunner had escaped to Shield World after the Halos were activated the first time.

Trivia

 * Shield World was referred to as "the Shield" of the Forerunner, while the Halos were "the sword." This is cryptically familiar to Cortana's line on the Halo 3 Announcement Trailer, "I am your shield, I am your sword."