Arcadia Forerunner ruins
From Halopedia, the Halo wiki
These Forerunner ruins were located in the jungles of the human colony world Arcadia. Located on the far side of the planet from Pirth City, they were the site of a conflict between United Nations Space Command and Covenant forces in the year 2531.
The ruins were located in a basin, overlooked by a plateau where an ONI facility resided. The visible portion of the complex consisted of numerous small structures surrounding a single central structure, which appears to have been an entrance to an underground facility. The smaller structures radiated from the main structure in a half-circle shaped pattern. The smaller structures were dark brown in color, possibly due to deterioration (similar to many of the structures on Installation 05), while the central one was covered in more conventional silver-gray Forerunner metal. All structures in the area had ornate patterns of shimmering blue lights. At least parts of the area were paved with characteristically patterned Forerunner material, which was largely covered in ground dirt after 100,000 years of neglect.
In 2531, the Covenant were directed to Arcadia by a star map on Harvest that pointed them towards the planet. The Covenant forces traveled to Arcadia with two CCS-class battlecruisers. One was destroyed by the UNSC battle group present in the system, but the other engaged in surface activity, attacking Pirth City and searching for Forerunner artifacts on the planet. After they found the ruins, the Covenant deployed a massive dome-shaped energy shield on the planet surface to prevent the UNSC forces from reaching the structures.
The UNSC forces established a base at the abandoned ONI base in proximity of the ruin complex, and managed to breach the energy barrier with a few Rhinos. After securing the area they established a Firebase and proceeded to investigate the area. However, the Covenant planned to keep the UNSC forces away from the main structure by closing off the entrance with a Super Scarab, which they were upgrading using technology obtained from the ruins. The Covenant attempted to destroy the UNSC base, by using the Scarab's focus cannon to destroy the small Forerunner structures in the way. However, the UNSC forces managed to weaken the Scarab by destroying its power nodes, and eventually the Scarab itself. After the Scarab had been destroyed, Professor Ellen Anders proceeded to investigate the area, under the watchful eye of Sergeant John Forge. While Forge and Anders were investigating the ruins, the Arbiter ambushed them, and captured Anders under orders of the Prophet of Regret, who hoped to force her to activate the Forerunner dreadnoughts at the Apex Site on Etran Harborage. Shortly thereafter, the Covenant ship jumped out of the system, with the Spirit of Fire in pursuit.
The now-defunct Halo Waypoint video series "The Halo Wars Story Retold" claims that the Covenant learned the location of Shield World 0459 from these ruins. According to Halo Wars: Official Strategy Guide, however, the Covenant had already discovered the shield world some time prior to the events of the game. The game's narrative seemingly supports the latter version: the shield world is merely used as the backdrop for the Prophet of Regret and the Arbiter's conversation about the events on Harvest and Arcadia, and no indication is given that the Covenant had only discovered the installation very recently.
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