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. The shield worlds were collectively known as "the Shield", while the Halos were collectively known as "the Sword".

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 Master Builder Faber'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.

Conservation sphere
The standard iteration of the shield world concept, known as the conservation sphere, consists of a planetary surface covering a miniature Dyson sphere. Beneath the artificial planetary surface span kilometers of machinery, including many access portals which lead to the sphere's interior, though remain sealed when not in use. The interior of the installation is hollow and features a vast terraformed surface and a large artificial star. At least one such installation, Shield 0459, had an interior diameter of 2,873 kilometers (1,787 miles).

While these installations provide safety from the firing of the Halo Array, many possess weapons that can be used to defend the installation's inhabitants, and some even harbor fleets of dreadnoughts. These defenses can be activated from an installation's control room or equivalent control center, which sometimes takes the form of an airborne platform; however, the control center must be activated by a human: a Reclaimer.

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

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 entrance to the interior is a small portal within the core of an artificial planet built around the core. Access to these shield worlds is possible only upon the imminent activation of the Halo Array.

The Dyson sphere within this type of shield world is enormous; the Sharpened Shield, for example, is two astronomical units (300 million kilometers) in diameter. As with the more common conservation spheres, micro Dyson sphere-type shield worlds feature vast terraformed surfaces and a single artificial star. The inner Dyson sphere resides within a slipspace bubble and is thus 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".

The Sharpened Shield
The Sharpened Shield, later designated "ONI Research Facility Trevelyan" by the UNSC, is a Dyson sphere formerly housed within a slipspace bubble of compressed dimensionality, having a diameter of only 23 centimeters in normal space. The slipspace bubble was located in the center of the artificial planet Onyx before the planet dissolved and the slipspace rift was deactivated. The sphere itself is approximately two astronomical units (300 million kilometers) in diameter and weighs approximately 1.37 solar masses. The Dyson sphere's default environmental conditions are suitable for Earth-based life, but the climate settings could be adjusted for other species' requirements. When Onyx was intact, the only means of access to the Sharpened Shield was a small portal, which was located inside the core room antechamber in the core of the artificial planet.

Near the end of the Battle of Onyx on November 3, 2552, Covenant Fleet Master Voro Nar 'Mantakree led his forces in an attempt to access the Sharpened Shield. However, SPARTAN Kurt-051 detonated a pair of FENRIS nuclear warheads within the core room antechamber after his comrades had passed through the portal, denying the Covenant access. The human survivors, including SPARTAN-II Blue Team, several SPARTAN-IIIs, Dr. Catherine Halsey, and Senior Chief Petty Officer Franklin Mendez, were then trapped within the shield world. Shortly thereafter, the installation's Sentinels destroyed the artificial surface of Onyx. After spending several days in the sphere — over three months in normal space-time — the survivors encountered a population of Huragok which had been left behind to maintain the sphere. They eventually manged to convince the Huragok into transitioning the shield world into normal space. Soon after, the Office of Naval Intelligence made the installation into a research facility in order to acquire the Forerunner technologies housed there.

Shield 0459
Shield 0459 was a standard conservation sphere: a hollow construct with a terraformed surface and an artificial star, surrounded by an artificial planetary shell. The interior of this construct could be accessed through one one numerous access tunnels around the surface. Like many shield worlds, Shield 0459 harbored a fleet of dreadnoughts for defense of its inhabitants. At some point, much of the planet's surface had been consumed by the Flood, giving it a barren, desert-like appearance.

During the Battle of Shield 0459 in February 2531, the crew of the encountered the Flood on the surface of and later within the shield world. Upon rescuing Professor Ellen Anders, whom Arbiter Ripa 'Moramee had captured in order to activate the installation's fleet of Forerunner dreadnoughts, the crew realized the danger this fleet could pose to humanity. Thus, they decided to sacrifice Spirit of Fire's slipspace drive, causing a supernova within the shield world's artificial star and destroying the vessels. This plan was successful: Shield 0459 was pulverized by the ensuing explosion, the dreadnoughts were destroyed, and all the Covenant forces and Flood forms on the installation were killed.

Trivia
Cortana's line in the Halo 3 announcement trailer, "I am your shield. I am your sword," echoes the Forerunners' reference to the shield worlds as "the Shield" and the Halos as "the Sword".

List of appearances

 * Halo 3
 * Halo 4
 * Halo Wars
 * Halo: Ghosts of Onyx
 * Halo: Cryptum
 * Halo: Glasslands