The Raid on High Charity also known as the Second Battle of High Charity, was a small engagement between the United Nations Space Command and the Fleet of Retribution against the Flood in late 2552 on the crashed planetoid of High Charity upon the surface of Installation 00.
Prior to the Second Battle of High Charity, the Flood had seized the Covenant mobile home planetoid of High Charity from the divided Covenant Separatists and Covenant Loyalists during the Battle of Installation 05 and the opening of the Covenant Civil War. A small UNSC force was present during the First Battle of High Charity, and although many UNSC personnel fell to the Flood, SPARTAN-II super soldier John-117 managed to escape, although the UNSC Artificial intelligence Cortana was left behind during the operation.
The Flood significantly Flood-formed High Charity, initially spreading metastasizing tendrils and biological growths throughout the planetoid and cutting power while spreading spores into the air, but later, after a short period, completely remodeled the planetoid. The doors were replaced with biological portas and the walls were decked with infested biomass with Flood Growth Pods and fleshy growths. It was no longer recognizable as a Covenant installation, but rather a Flood hive from the interior. All remaining Covenant on the planetoid were killed and/or infected. Cortana surrendered to the Flood for her own safety and was tortured for information, almost succumbing.
The Gravemind, with his newly found space station and billions-trillions of Flood forms, planned to attack Earth, the UNSC/Covenant Separatist stronghold, and infest it with Flood. However, there he found a Forerunner Portal which led from Earth to Installation 00: the Ark, a center from which all the Halo Installations could be simultaneously and remotely fired. Sensing that was the greater threat, the Gravemind slipped High Charity through the portal, sparing Earth instead for the Ark. However, during the Slipspace transition, High Charity emerged in the fray of the Battle of Installation 00, and parts of it crashed into the Fleet of Retribution Assault Carrier Shadow of Intent. Parts of Flood-infested High Charity were sheared off and crashed into the installation below while the Assault Carrier was crippled. However, High Charity crashed into Installation 00's surface, apparently from damage from the Slipspace transition possibly combined with structural weakening from High Charity's transformation into a flood hive or poor piloting that is the norm for the Flood. While Flood exited and spread across the Ark, infesting the Citadel and also the Control Room, large amounts of Flood still took residence within the crashed hulk.
On the Ark, however, its forges were manufacturing a new Halo Installation to replace the lost Installation 04 which was destroyed earlier on in the year. The Covenant Separatist/UNSC alliance realized that in order to contain the Flood outbreak on Installation 00 spread by the crashed High Charity, they needed to flee and then activate the Halo, sterilizing the Flood on the Ark. However, Cortana, the lost UNSC Artificial intelligence still within High Charity, was the sole owner of the Index from the first Installation 04 which was necessary to fire the Halo installation. While Sergeant Major Avery J. Johnson and the Arbiter organized a systematic Separatist/UNSC withdrawal from Installation 00, SPARTAN-117 ventured into the crashed High Charity via Banshee fighter to retrieve Cortana and rescue her, and also secure the Index to fire the incomplete Installation 04 replacement being built on the Ark.
SPARTAN-117 entered the crashed High Charity through a hole in the planetoid's surface in the western part of the station, and then proceeded inwards to the center in a quest to find Cortana. At first, he met only light resistance, with only the Infection Forms attacking. However, the Flood began to take notice of his intrusion, sending waves of Stalker Pure Forms to search for him and then deploying Combat Forms and Tank Pure Forms to assail him with Carrier Forms lumbering behind and Ranged Pure Forms on the walls opening fire on the SPARTAN while closer-ranged Flood attacked him directly. The strife began to intensify because of the numbers of Elite and Brute corpses scattered on the ground, which were easy targets for reanimation from the massive numbers of Infection Forms, and also because of the closely-packed claustrophobic tight quarters, whose walls were pulsing with Flood biomass.
Not long after the Flood took note of SPARTAN-117's intrusion, the Gravemind began to take notice of him, contacting him in enigmatic and disorienting transmissions. At first, the Gravemind was demanding an answer as to why the SPARTAN had come to High Charity, wondering if SPARTAN-117 "sought forgiveness" for his father's sins. After a minute, he realized that SPARTAN-117 was seeking Cortana. He then began boasting that he had somehow merged with Cortana, and that together they were "two corpses in one grave." As John-117 forced his way deeper into High Charity and closer to Cortana, the Gravemind began getting irritated by his progress, shouting at him and making fierce threats such as "Submit! End her torment and my own!" and "You will tell me what she hides! Or I will feast upon your bones!" This was accompanied by loud growls, perhaps in an attempt to "scare" the Spartan away. Cortana began to make intense transmissions to SPARTAN-117 where it appeared that she was rampant; her voice began to fracture into voices of different pitch and she adopted a chilling high-pitched giggle as she rambled in incoherent words.
However, moving his way inwards, SPARTAN-117 managed to navigate a tall infested chamber, reaching the top and making it to one of High Charity's secondary reactor rooms.
- “You found me. But so much of me is wrong, out of place. You might be too late.”— Cortana
Upon reaching the next room John-117 encountered Cortana, who was trapped in a Stasis Field. He broke the field, finally fulfilling his promise to return and retrieve her after his personal war with the Prophet of Truth and Covenant had concluded. Cortana revealed that she had maintained her copy of the Index from the original Halo, and that it would function in activating the unfinished replacement. Following this revelation, Chief affirmed that he was finishing the fight, and his companion returned to her iconic data chip, which SPARTAN-117 slotted inside his helmet. The two then made their way back to the reactor room, where Cortana told John-117 to destroy High Charity to buy them more time to activate the replacement Halo. John-117, fighting his way through innumerable Flood forms, destroyed the reactor pylons, destabilizing the city-ship's primary power source. As the massive vessel began to crumble around them, the two made their way through the exploding city and encountered the Arbiter, who had come to help. At the last moment, the trio found a functional Pelican and escaped High Charity as its reactor went critical, destroying the lesser Flood forms aboard; despite the efforts of both sides of the Great Schism and the UNSC, Gravemind survived by escaping to the replacement Halo to begin again the process of constructing its body.
- The corpses littering the level could be viewed as a design mistake; any viable biomatter on High Charity was being consumed by the Flood incursion for the purpose of creating additional Combat Forms to boost their numbers. It is also possible that the corpses, like that of Marvin Mobuto on Installation 04, were too damaged to be utilized by the Flood; in contrast to this, they may have been harvested from the battlefield by the Flood for the purpose of nourishing the Gravemind, which had launched the parasitic assault on the city-ship some days before when it arrived aboard the UNSC In Amber Clad. As for the close proximity of Brute and Elite corpses, it can be speculated that they engaged in mutually assured destruction during the First Battle of High Charity or, again, that their bodies were moved together in death regardless of their rivalry in life by the Flood.