You may be looking for the Halo Online map, the Requiem site codenamed the Refuge or for the Halo 3 achievement of the same name.
According to Enduring Bias, Shield World 0673 was the last shield world to be constructed by the Forerunners and was apparently never fully finished. However, the monitor's statement may not be fully accurate due to the compartmentalization protocols limiting its knowledge base.
Decades before the Covenant's formation, a Sangheili known as 'Crecka discovered the shield world while fleeing assailants of an unidentified alien species in the System of Miasmic Giants. 'Crecka entered the world and encountered Enduring Bias, who urged that the Sangheili submit himself for study and give the ancilla information about his species. However, 'Crecka escaped and the monitor did not take all available measures to stop him as its programming had deteriorated considerably over the tens of thousands of years of overseeing the installation.
In 851 BCE, 'Crecka led Ussa 'Xellus, the leader of a Sangheili resistance movement against the budding Covenant, as well as his mate Sooln, to Shield World 0673 aboard 'Xellus' ship Clan’s Blade. The trio entered the shield world through an access portal and made their way to the terrestrial interior of the world where they were greeted by Enduring Bias. Recognizing the Sangheili as a species indexed by the Librarian in the Conservation Measure, the Forerunner AI offered shelter for Sangheili although it demanded to know their intentions.
Ussa 'Xellus intended to use Shield World 0673 as a refuge for his rebel movement until they could gather sufficient strength to strike against the Covenant. He put this plan into action upon returning to his hideout in Nwari, Sanghelios. Not long after the Ussans had settled in the shield world, its location was compromised to the Covenant because of a traitor. A Covenant fleet arrived in the system and the Prophet of Inner Conviction attempted to personally convince Ussa 'Xellus to surrender. However, 'Xellus instead activated the shield world's disassembly mechanism, causing it to break apart explosively. While Inner Conviction suspected the rebels may have survived, he withdrew the fleet and had the event recorded as a mass suicide on the Ussans' part.
The Ussan faction continued to inhabit the disparate components of the shield world, scattered across the system's asteroid belt, for over 3,000 years, forming their own civilization that significantly diverged from the mainline Sangheili over that time. In 2553, they were discovered by a group of Sangheili-led Covenant fleeing the fall of High Charity. By this time the sections had dilapidated significantly and the Sangheili population was plagued by dissidents as well as a disease known as the Blood Sickness, caused by faulty protein synthesizers. However, thanks to the arrival of a Huragok, Sluggish Drifter, Enduring Bias, who had been damaged in the disassembly process, was made operational again. The monitor helped Kaidon Bal'Tol 'Xellus neutralize the rebellion that was threatening the integrity of the colony and reconnected the surviving portions of the shield world; they were not sufficient to form a full sphere again but instead formed a small ringworld somewhat similar to a Halo ring.
After their discovery, the Ussans returned to contact with the Sangheili at large and those willing to return to Sanghelios were given the opportunity.
Shield World 0673 featured a silver-gray metallic exterior surface encasing a hollowed-out planetoid. The rocky planetoid's surface—known as the "eco level"—possessed a habitable terrestrial environment filled with transplanted species. Illumination was provided by an artificial star, its light reflected by shafts in the ground as well as panes in inverted, stalactite-like structures in the shell that surrounded the planetoid—the eco level's ceiling. The shield world's inner workings were housed within the hollowed-out planetoid, beneath the surface of the eco level.
As a capability unique among shield worlds, the Refuge was equipped with a failsafe measure in which the world broke apart explosively and dispersed the components of its outer shell across the system's asteroid belt in the event the installation was compromised. The small fragments of the world would be able to avoid detection among the asteroids before it was safe to reconstitute the shell. However, this measure was highly risky to lifeforms on the world.
The shield world was known to have at least one rectangular entrance portal, through which the Sangheili gained access inside. Once the spacecraft had entered the portal, walls and a ceiling formed around the ship; the interior of the newly-formed room soon pressurized and a door opened. After a series of descending corridors, visitors would arrive on a platform overlooking the eco level. The platform would then descend down, providing access to the ground level in an area described as looking "too haphazard to be a cultivated garden, but too orderly to be wilderness."
Various Forerunner automatons, including airborne freight transports, moved about in the interior.
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