Shield world

A Shield World is a Forerunner installation, designed to be a shelter from the activation of the Halo Array. The main component of a Shield World is a complete and thorough Micro Dyson Sphere, located within a Slipspace bubble of compressed space-time. The only way in is a small portal that constitutes the only real-space component of a Shield World. Access to Shield Worlds was only possible upon the imminent activation of the Halo Array.

Description
The Dyson Sphere within a Shield World is two astronomical units, ie. 300 million kilometers in diameter, and its inner surface is entirely terraformed and capable of supporting life. In the center is an artificial star. The inner Dyson Sphere resides within a Slipspace transit, accessible only from the Core Room Antechamber in the heart of the artificial planet formed around the structure. In normal space, the slipspace bubble is radically smaller than the interior, being only a few meters in diameter.

History
The Shield Worlds were built by the Forerunners in their vast empire, as refuges from the Halo Array's pulse. However, the betrayal by the Forerunner AI Mendicant Bias and its defection to the Flood forced the Forerunners to abandon their plans to evacuate into the shield installations, as Mendicant Bias revealed the locations of the Shield Worlds to the Flood. Because of this, when Halsey and company fled into Onyx's Shield World during the last stages of the Battle of Onyx, the Shield World was empty.

Caches of Technology
An alternate type of the Shield World exists. While referred to as Shield Worlds, these facilities are considerably different from the base concept. Instead of having one single access portal open for a limited time, and the Dyson Sphere beyond a slipspace transit inside a bubble of compressed time-space, these installations are more conventional in design. Whether they were built for the same purpose as their larger counterparts, is unknown. The artificial planet is hollow, with an interior diameter of approximately 3000 kilometers. The planet's inner surface is terraformed; this makes up the Dyson Sphere. Multiple access tunnels are located around the planet, allowing access to the interior at will.

The Covenant owe their rapid technological advancement to the discovery of a small number of these worlds, discovered earlier in their history and stripped of usable technologies. The eventual fate of these worlds, and whether they were abandoned or inhabited by the Covenant, is unknown.

One of these worlds contained a fleet of Forerunner warships, which the Covenant planned to use in their war against the humanity. Before they could accomplish this, the crew of the UNSC Spirit of Fire destroyed the Shield World by overloading the ship's Shaw-Fujikawa Translight Engine in the sun in the center of the shield world. This caused it to go supernova, thus turning the entire structure into dust, preventing the Covenant from using the Forerunner fleet.

Onyx
The internal Dyson Sphere, named The Sharpened Shield, resides in a Slipspace bubble of compressed dimensionality, having a diameter of only a few meters in normal space, but vastly larger inside the slipspace rift. The Dyson Sphere contains environments suitable for Earth's sentient life. The only way in is a small portal, which is located inside the Core Room Antechamber in the core of the artificial planet.

Flood-Controlled Shield World
A "technology cache"-type Shield World, this world differed greatly from the base concept of the Shield Worlds, being only a hollow artificial planet with a terraformed interior, instead of having a massive Dyson Sphere inside a slipspace rift. Accessing the interior was also easier than on Onyx, as the Shield World had multiple access tunnels around the surface. Its function was also apparently different, housing a fleet of Forerunner Ships instead of being a bomb shelter.

Trivia

 * Worlds like Onyx were 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." In the Halo Encyclopedia, the Micro Dyson Sphere inside Onyx is referred to as "The Sharpened Shield".
 * A "shield installation" is mentioned in the cutscene on the level "The Ark" in Halo 3. 343 Guilty Spark states that he had hypothesized that the Ark might be located within such an installation, but was proven wrong by the discovery of its location on the exterior of the galactic rim. This also brings up the possibility of many Shield Worlds existing, further assisted by the Flood-controlled Shield World that appears in Halo Wars
 * In Terminal 5 of Halo 3, there is a sentence that reads the following "If we start immediately--commence total biosphere elimination of life sustaining worlds (as indicated in the accompanying charts) and relocate evacuated populations to facilities such as those described in the [Onyx project] all this could be achieved in [57,1590 (+/-2,184) hours]." This is most likely a reference to either the Forerunners Shield World(s) plan or Onyx itself.
 * Unlike the Halos, it is unknown how many Shield installations exist. When the Forerunners built the Halos, it is possible they built an accompanying shield world with each. The Halos would destroy all sentient life within a 25,000 light year radius of itself while the Shield World would protect any sentient life within that region.