Trove

"But this world lies dormant. It is just a shell!"

- The Arbiter

This Flood-controlled Shield World was a world deep in Covenant space. An artificially constructed Forerunner installation, it was a variation upon the Shield World concept, originally intended to protect sentient lifeforms from the effects of the Halo Array. It also served as a port for a fleet of Forerunner Ships.

Summary
The installation's purpose was to protect lifeforms within it from the Halo Array and The Flood, and also serve as a port for a massive fleet of Forerunner ships. Despite the fact that the Shield Worlds were created as a haven from the Array and the Flood, the external surface was heavily infested with Flood biomass, with forms of various types freely roaming across the surface. Sentinels were a rarity on the outside, but this changed within the center, where the many Forerunner constructs worked to prevent the Flood from claiming the interior as well, without much effect.

Structure
The Shield World's main structure consisted of the outer surface, the superstructure and tunnel system inside the crust, and the inner Micro Dyson Sphere, along with it's own miniaturized artificial sun. The outer surface was entirely infested with the Flood, as well as parts of the superstructure. Some Flood even reached to the interior of the world, but because of the larger numbers of Sentinels, never managed to fully infest it.

There were several access portals around the surface, which could be used to gain access to the Shield World. Most of them were likely hidden, for example covered under large bodies of water, such as the one The Spirit of Fire used to enter the Shield World. The access portals were seemingly automated, and controlled by an AI. Within the tunnels inside the superstructure, there was a cleansing system, which made sure that no Flood entered the interior. The cleansing system consisted of a "Cleansing Ring", which created energy fields within their diameter, which would destroy any Flood mass on a passing ship.

The inner Dyson Sphere was completely terraformed and full of plant and animal life, much like the Halo installations or The Ark. It had numerous structures and systems within it, ie. The Apex. In the center of the Micro Dyson Sphere there was a miniaturized artificial star. Like many other Forerunner installations, this Shield World possessed a Teleportation Grid.

History
"The treasures here and the bounty they will bring are part of a larger plan. We must not waiver."

- Prophet of Regret

This Shield World was discovered by the Covenant many years prior to the Human-Covenant War. While the Covenant could access it, they weren't able activate it's most important installation: The Apex, which contained a massive fleet of Forerunner ships, locked in place and unreachable with Covenant technology.

After the war with the humans began, the Covenant hoped to use the Shield World's stored Forerunner warships to crush the UNSC quickly, rather than wage the protracted war that would characterise the Human-Covenant War. The need to find a key to unlock the world had taken precedence over all other tasks, even the search for the rings.

When the Covenant were searching to find a key to activate the Shield World's installations, they came across a Relic on Harvest, a human colony they had recently glassed. The Prophet of Regret hoped this would finally be the key to activate the Apex installation, and sent the Arbiter to secure the relic, and find the key.

However, the structure on Harvest was revealed to be something different: A map room of nearby star systems. The map led the Covenant, and the UNSC support vessel, in pursuit, to Arcadia, another human colony. What they found there is not known for sure, but seemingly they didn't find the key to activate the installation.

While the UNSC and the Covenant forces were fighting on Arcadia, the Arbiter had travelled back to the Shield World. The Prophet of Regret ordered the Arbiter to go to Arcadia and capture Professor Anders, as she seemed to have unusual knowledge about the Forerunners. The Arbiter went back to Arcadia, captured Anders and brought her to the Shield World. The Spirit of Fire followed in close pursuit, tracking Anders' transponder signal. When the Spirit of Fire arrived to the Shield World, a new horrific enemy was discovered: the Flood. Some time later, when the Spirit's shipboard AI Serina scanned the planet for Anders' signal, which seemed to be originating from an ocean, the ship was suddendly pulled into the Shield World's internal docking mechanism. After passing the tunnels inside the installation's superstructure, the Spirit of Fire emerged into the internal Micro Dyson Sphere.

Destruction
"Well, put simply, Captain, our reactor will cause the sun to go supernova, leaving nothing here but dust."

- Ellen Anders to captain James Cutter.

Some time later, the Arbiter used Anders, who had been kept prisoner withing the Apex, to activate the fleet of Forerunner ships. However, as the awestruck elites watched the fleet's activation, Anders managed to escape via the installation's Teleportation Grid and regroup with the other humans. She quickly came up with a plan to prevent the Covenant from using the Forerunner fleet: They overloaded the Spirit of Fire's FTL Reactor to create a chain reaction in the artificial sun in the center of the Shield World, destroying it from within.

Comparison with the Onyx Installation
Unlike Onyx, which was made up of trillions of Sentinels, from which a planetary surface had formed, and a massive Dyson Sphere residing in a compressed Slipspace bubble in the center of the construct, this Shield World was far simpler: A hollow structure with planetary surfaces on the interior and exterior.

The internal Micro Dyson Sphere was somewhat similar to Onyx's Dyson Sphere, being terraformed and possessing an artificial sun, though it and the entire structure were on a far smaller scale. However, unlike Onyx, it was not designed as a bomb shelter from the Halo Array, but as some sort of harbor, shipyard or hiding place for ships, as it contains The Apex.

While this Shield World appears to be approximately 2900 kilometers in diameter, Onyx's internal Dyson Sphere's diameter is approximately 150 million kilometers, and therefore immensely larger than this installation.

As this Shield World had dozens of Forerunner ships stored within it, it is unknown whether the same is true for the Onyx Installation.

Trivia

 * In the cutscenes that show the surface of the Shield World, what appear to be Forerunner cities can be seen across the land.
 * While in the pre-rendered cinematics the lines on the inner surface of the shield world are latitudinal, they are longitudinal in-game in the level Repairs. The appearance of the surface is also different. However, this is probably just due to in-game graphical limitations.
 * According to a hologram of the Shield World in the final cinematic of Halo Wars, the distance from an exit portal to the sun in the center is 1,437,572,31 meters (1437,6 km). This makes diameter of this Micro Dyson Sphere to approximately 2875 kilometers.
 * The concept of a hollow planet, with a habitable inner surface, is similar to that of a Globus Cassus, a proposal to hollow out the Earth.

Related Links

 * The Apex
 * Micro Dyson Sphere
 * Shield World