Trove

This Flood-controlled Shield World was a world well outside the spheres of influence of both the UNSC and the Covenant. 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.

The Covenant secured a relic on Harvest, pointing them towards another UNSC colony, Arcadia. Another relic directed them to the location of the Shield World, where Ripa 'Moramee 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. To prevent this, the sacrificed its FTL Reactor to create a chain reaction in the artificial sun in the center of the Shield World, destroying it from within.

Despite the fact that the Shield Worlds were created as a haven from the Array and the Flood, the external surface is heavily infested with Flood biomass, with forms of various types freely roaming across the surface. Sentinels are a rarity on the outside, but this changes within the center, where the many Forerunner constructs work to prevent the Flood from claiming the interior as well, without much effect.

Comparison with the Onyx Installation
Unlike Onyx, which was made up of trillions of Sentinels, from which a planetary surface had formed, this world was built in a method similar to a Halo Installation, though on a much larger scale, being a hollow structure with planetary surfaces on the interior and exterior being supported by numerous pillars. Inside this is a more familiar construct of a ship-sized portal allowing access from an opening on the outside to the interior world. This Shield World is much like Onyx, possessing an artificial sun, though it and the entire structure are on a far smaller scale. However, unlike Onyx, it was not designed as a bomb shelter, but as some sort of harbor, shipyard or hiding place for ships, as it contains The Apex. Also, the Flood Controlled Shield World had dozens of Forerunner ships stored within it, it is unknown whether the same is true for the Onyx Installation.

It is possible that the entire Shield World in Onyx was a Shield World just like in Halo Wars, and the Forerunners somehow placed the whole planet into Slipspace. It could be possible that the Shield World in Halo Wars was meant to be placed into Slipspace and then a planet made of Sentinals was to be built around it until the Shield World was infested with the flood.

Trivia

 * In the cutscenes that show the surface of the Shield World, what appear to be Forerunner cities can be seen across the land.