Trove

This Flood-controlled Shield World was a Forerunner Shield Installation deep in uncharted space. A variation upon the Shield World concept, originally intended to protect sentient lifeforms from the effects of the Halo Array and the Flood, it also served as a port for a 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.

During the Human-Covenant War, the Shield World was discovered by the Covenant during the First Battle of Arcadia, and subsequently destroyed in the Battle of the Flood-controlled Shield World in February 2531.

Structure
This 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, as well as parts of the superstructure, were largely infested with the Flood. Some of the Flood even reached to the interior of the world through openings like vents, but because of the larger numbers of Sentinels, never managed to fully infest it.

The ecosystems of the interior and the exterior were radically different, the outer surface being a rocky wasteland with large bodies of water and Flood biomass in some areas. The atmosphere was thick with dust, which may have been a defensive measure to disable starships and prevent them from carrying the Flood off-world. Parts of the superstructure were also seen on the surface, with chasms reaching deep down into the interior.

The inner Dyson Sphere was completely terraformed and full of plant and animal life, much like the Halo installations or The Ark. It also featured different types of Forerunner structures, which in many places were more prominent on the surface than on other Forerunner installations. In the center of the Micro Dyson Sphere there was a miniaturized artificial star. The inner surface had latitudinal lines of structures encircling the sphere. Like many other Forerunner installations, this Shield World possessed a Teleportation Grid.

Access Tunnels
The surface of the Shield World had multiple access portals, 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 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 prevented the Flood from entering the interior. This quarantine system consisted of several "Cleansing Rings", which created energy fields within their diameter, which would destroy any Flood biomass on a passing ship. On the interior, these portals had six interlocks that needed to be opened in order to unlock the access to the tunnel, and they could only be operated by a human.

The Apex
The Apex site was an installation comprised of a control center on a platform high above the inner surface of the Shield World, and numerous docking towers containing a fleet of Forerunner Ships. The control center was accessible from the ground by a teleporter referred to as Apex Base site, which was on top of a plateau with a series of ramps leading up to it.

History
"This world shall be the birthplace for an invasion fleet beyond imagining!"

- Prophet of Regret

The Shield World was discovered by the Covenant in 2531 using information from a Forerunner complex on the planet Arcadia. After recovering the information, the Arbiter Ripa 'Moramee and the Prophet of Regret traveled to the Shield World, to discover the Apex site and the dormant Forerunner fleet. With the Human-Covenant War ongoing, the Covenant hoped to use the Shield World's stored warships to crush the UNSC quickly, rather than wage the protracted war that would characterize the Human-Covenant War. However, they soon discovered that while they were able to access the installation, they weren't able to reactivate the fleet.

After the Covenant had observed professor Ellen Anders activating the Forerunner relic on Harvest, the Prophet of Regret ordered the Arbiter to go to Arcadia and capture the human scientist, 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 the Covenant 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 suddenly 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.

"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 within 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 Shaw-Fujikawa Translight Engine to create a chain reaction in the artificial sun in the center of the Shield World, sending it to supernova state and destroying it from within. The Spirit of Fire was able to escape the Shield World's destruction.

Comparison with the Onyx Installation
Unlike Onyx, which was made of trillions of Sentinels, upon which a planetary surface had been formed, and a massive Dyson Sphere residing in a compressed Slipspace bubble in the center of the construct, this Shield World was far simpler. This Shield World was comprised of 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 apparently not designed only as a shelter from the Halo Array, but also as some sort of harbor, shipyard, or hiding place for ships, as it contained The Apex. While this Shield World was approximately 2900 kilometers in diameter, Onyx's internal Dyson Sphere's diameter is approximately 150 million kilometers.

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 inner surface of the Shield World, lines of Forerunner structures can be seen across the land. These could be Forerunner cities.
 * 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.
 * The concept of a hollow planet with a habitable inner surface is similar to that of a Globus Cassus, a proposal to use the Earth as building materials to build an enormous hollow megastructure around Earth.
 * Derelict Forerunner ships can be seen on the outer surface of the Shield World, likely indicating a battle between the Forerunners and the Flood.
 * It is likely that the Flood on the Shield World, prior to the Covenant's arrival, were comprised of biomass from Forerunner bodies.

Related Links

 * The Apex
 * Micro Dyson Sphere
 * Shield World