The Battle of Installation 05, also known as the Battle of Delta Halo, was a major engagement between the United Nations Space Command, Covenant, Sentinels, and the Flood on the Forerunner ringworld Installation 05, otherwise known as "Delta Halo".
Introduction
During the First Battle of Earth, the Prophet of Regret lead a small fleet of two Assault Carriers and thirteen CCS-class Battlecruisers on an expeditionary raid that stumbled upon Earth, the strongest UNSC stronghold planet. After substantial ground, air, and space losses, Regret fled in an Assault Carrier over New Mombasa, opening a Slipspace rift and destroying the city in a devastating Slipspace cataclysm as it initiated an emergency jump to Installation 05, although how the Covenant obtained the coordinates to Installation 05 were unknown.
However, upon Fleet Admiral Hood's order, five United Nations Space Command Defense Force warships nearby Mombasa were close enough to catch onto the Assault Carrier's rift before it terminated: three UNSC Frigates, the UNSC In Amber Clad, UNSC Paris, and the UNSC Redoubtable, one UNSC Corvette, the UNSC Coral Sea, and one UNSC Prowler, the UNSC Dusk, were able to enter the Slipspace field, trailing the Assault Carrier.
Their shadowing, however, was not exactingly precise. Only the frigate In Amber Clad was successfully trailed the Assault Carrier to Installation 05. The prowler Dusk arrived days later near the Covenant homeworld ship High Charity, and the Coral Sea, Paris, and Redoubtable were not anywhere near the engagement because of the imprecise physics of Slipspace.
Fortification
While Regret's assault carrier hovered over the installation, deploying Wraiths, Ghosts, Banshees, and also companies of Elites, Jackals, Drones, and Grunts by means of Phantom dropships. The Covenant ground forces fortified themselves in a series of Forerunner ruins and temple structures, even establishing Shielded Plasma Cannons for further defense.
Meanwhile, the In Amber Clad trailed behind, and after the Assault Carrier moved off, Commander Miranda Keyes, captain of the In Amber Clad, ordered a platoon of Orbital Drop Shock Trooper special forces operators and SPARTAN-117 to establish a landing zone on Installation 05 to allow Sergeant Avery J. Johnson to bring in two flights of Pelicans and further expand the UNSC presence. The frigate made a quick pass-over of a segment of the ringworld, deploying a dozen Human Entry Vehicles. Six survived it into the atmosphere of the ring, and only four personnel eventually survived - SPARTAN-117 and three "Helljumpers". However, before they could land in the small ruins, they were detected by the Covenant, and SPARTAN-117 led the commandos in eliminating Covenant plasma turrets and holding the ruins against waves of Covenant troops until a Pelican could drop off an M12 Warthog LRV and a pair of Marines.
While the immediate objective for SPARTAN-117 and his Marines were to push through the Covenant-held Forerunner ruins, the overall UNSC objective for Installation 05 was to ensure that the Covenant would not activate the ring and extinguish all sentinent life in the galaxy. To do this, the UNSC had to infiltrate the Library, retrieve the Index, and safekeep it, thereby preventing the Covenant from using the Index to fire the ring.
After SPARTAN-117 and his men pushed through two Forerunner ruins, another Pelican air-dropped Resupply Canisters and a small number of Marines, refueling the UNSC offensive as they advanced through a ravine defended by Jackal Snipers and Drones, and as they reached another temple - this one with a hologram of the Prophet of Regret, who was sermonizing in a main Forerunner temple, preparing for the activation of the ring and the sublimation event of the "Great Journey". His elimination became the primary objective for SPARTAN-117, as his death would destabilize the Covenant politically and religiously. After Cortana deciphered the hologram, SPARTAN-117 and a pair of Marines battled through a pair of Covenant-held temples before reaching an open platform. At the time, the Covenant became aware of UNSC activity, and began becoming aggressive, so Commander Keyes sent several Pelicans towards the Master Chief, which air-dropped more Resupply Canisters on the platform.
The UNSC continued attacking, proceeding from the platform to gondola which took SPARTAN-117 and his remaining Marines to a heavily-defended Forerunner underwater temple which was guarded by Hunters, Stealth Elites, and Honor Guard Elites, amongst other Covenant forces. After emerging from the submerged temple, they emerged at a hilly Forerunner ruin, where Drones, Elites, and Grunts attempted to halt the UNSC advance, and Phantoms arrived with additional Covenant reinforcements.
However, the Covenant were ultimately unsuccessful: the UNSC, after clearing out the last ruins, were able to secure an LZ for a Pelican to air-drop Resupply Canisters and a team of Marines, who would aid SPARTAN-117 in the final push towards the main temple. They took a gondola towards the temple, which was attacked by Banshee fighters and Rangers, but they were still unable to prevent