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 is only possible upon the imminent activation of the Halo Array but have been known to open on other occasions such as a Reclaimer being present.

Description
The Dyson Sphere within a Shield World is two astronomical units, i.e. 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 05-032 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.

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. Still, they serve the same purpose of "bomb shelters" as their larger counterparts. 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 owes their rapid technological advancement to the discovery of a small number of these worlds, discovered earlier in their history and stripped of useful resources. 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 humanity. Before they could accomplish this, the crew of the 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 (Trevelyan)
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 being the size of a solar system 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 Onyx. It was renamed Trevelyan in honor of SPARTAN Kurt-051, who sacrificed his life so others could enter the shield world, by request of Chief Mendez. Trevelyan was Kurt's surname before he was abducted for the SPARTAN program.

It was called the "Sarcophagus" by the Forerunners.

Shield 0459
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 functionality was expanded as well, as it housed a fleet of Forerunner Ships and other technologies in addition to being a shelter from the Halo Array's pulse.

Requiem
"Instead of surrounding a star, the massive exterior shell of Requiem encases a planet - a variation on a Forerunner Shield World". Seen in multiple trailers of Halo 4 and the legendary ending of Halo 3, the interior appears to be at least in part natural, complete with clouds and greenery. The entrance portal shown is also evidently not overly secure, with the UNSC Infinity, the remains of the UNSC Forward Unto Dawn and Covenant forces able to pass through and deploy ground forces. Requiem is also inhabited by various Forerunner AI, which, unlike Sentinels, seem to be combat specific rather than the Sentinels of the Halos which are basically prison guards to contain the Flood. Such AI include the Promethean Knight, Promethean Crawler and Promethean Watcher. While Sentienels no longer act as the main guards, they are seen on Requiem in various chambers.

Trivia

 * Worlds like Onyx were referred to as "The Shield" of the Forerunner, while the Halos considered "the Sword." This echoes Cortana's line on the Halo 3 Announcement Trailer, "I am your shield, I am your sword." So far, Bungie has said nothing about a correlation between Cortana's use of the word "Shield" and the Forerunner's "Shield World."
 * 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 25000 light year radius of itself while the Shield World would protect any sentient life within that region.

List of appearances

 * Halo: Ghosts of Onyx
 * Halo Wars
 * Halo: Glasslands
 * Halo 4