Shield world

A Shield World is a Forerunner installation designed as a military outpost to monitor and counter potential Flood outbreaks, as well as to serve as a shelter from the activation of the Halo Array. Scattered across the Forerunner ecumene, the Shield Worlds outnumber the Halos by a large margin.

History
Over a thousand years prior to the second coming of the Flood, The Shield Worlds were proposed by the Didact as facilities from which to fight and research the Flood, an alternate solution to the Master Builder's plan to construct the Halos. However, the betrayal by the Forerunner 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.

Known types
While they share their name and basic function, there are two known types of Shield World drastically different from one another.

Micro Dyson sphere
The main component of this type of Shield World is a Dyson sphere located within a bubble of compressed slipstream space. The only way in is a small portal within the core of an artificial planet built around the core. Access to these Shield Worlds was only possible upon the imminent activation of the Halo Array.

The Dyson sphere within this type of Shield World is two astronomical units (300 million kilometers) in diameter, and its inner surface is entirely terraformed and is 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, hence the moniker "Micro Dyson sphere".

Onyx is the only known construct of this type.

Planetary shell type
Another type of Shield World exists, which is apparently more common than the aforementioned type and likely represents the standard version of the Shield World. These facilities are considerably different from the Onyx installation. Instead of having a single access portal that is open for a limited time, and the Dyson sphere being located 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 3,000 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, which they discovered earlier in their history and stripped of usable technologies. The eventual fate of these worlds, and whether they were abandoned or remained inhabited by the Covenant, is unknown.

One such Shield World 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 the sun to go supernova, thus disintegrating the entire structure and preventing the Covenant from using the Forerunner fleet.

Onyx
Onyx's 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
This world was a hollow artificial planet with a terraformed interior, instead of having a full-sized 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 warships and other technologies in addition to being a shelter from the Halo Array's pulse.

Trivia

 * The Forerunners referred to Shield Worlds as "the Shield", while the Halos were "the Sword." This echoes Cortana's line in the Halo 3 announcement trailer, "I am your shield. I am your sword."

List of appearances

 * Halo 3
 * Halo Wars
 * Halo: Ghosts of Onyx
 * Halo: Cryptum