The Battle for the Hephaestus Array, also known as Operation: SWEEPOUT, was an engagement between United Nations Space Command, United Rebel Front, and Covenant forces. Starting out as what was considered a routine counter-insurrectionist operation to the colony world of Endrin, it would soon blossom into a short but brutal battle above the planet's surface against a partially-constructed rebel superweapon that worsened as Covenant forces joined the fray.
Though the UNSC would eventually triumph, victory came at a heavy cost, with only a few ships remaining and two irreplaceable SPARTAN-II's lost. Nonetheless, a heavy blow was struck against the United Rebel Front, depriving them of one of their best commanders for a time and prompting a change in leadership in one of its factions.
In late November of 2550, several merchant ships had disappeared in the area and communication with the colony was lost, though little attention was paid to the strategically insignificant system until the hijacking of a vessel belonging to the Office of Naval Intelligence nearby forced local UNSC forces into action. The ship, carrying several experimental suits of MJOLNIR powered assault armour, had been tracked to the Olmarra system before it was destroyed. Unwilling to allow such crucial equipment to fall into enemy hands, Battlegroup Tempest was dispatched to investigate, with Rear Admiral Gregory Kesson leading it. Attached to the battlegroup were five SPARTAN-II's belonging to Sigma Team, who were assigned with personally retrieving the stolen suits once their location was discovered.
As the UNSC forces drew closer to Endrin, they were attacked by a smattering of small but well-armed ships that had been lying in wait close to one of the planet's moons. All but one was quickly destroyed by the battlegroup, with the final ship fleeing towards the other side of Endrin. As the UNSC Livingston pursued it, the destroyer was struck by seven consecutive MAC blasts and destroyed. Tracking the source of the strikes and believing a rebel fleet to be lying in wait, Kesson's ships soon found themselves facing a massive construct, half-space station and half-asteroid, in Endrin's orbit. Once they were in sight, the battlegroup received a warning from a rebel leader commanding the structure, ordering them to surrender or be destroyed.
Believing the rebel's surprise attack to have been a fluke, Kesson chose to attack, and split Battlegroup Tempest up to divide enemy attention. While the Insurrectionist construct did possess a great deal of firepower, with a rudimentary network of MAC guns and point-defence turrets that kept the UNSC ships on the defensive as a second volley destroyed two frigates, its unfinished state left it highly vulnerable to boarders. Over forty Pelican dropships were deployed, carrying battle-hardened Orbital Drop Shock Troopers and part of Sigma Team to secure the station while the battlegroup's Marine contingent landed on Endrin itself, having located a sizeable rebel encampment. Many dropships were lost as they ran a gauntlet of anti-aircraft fire, but most of the attack force arrived safely and boarded the station, identified by its defenders as the 'Hephaestus Array'. Leading the assault were Spartans Jacob-076, Wulf-041 and Marco-035, who rightly suspected that the Insurrectionist leader who contacted them was none other than Jack-085, a SPARTAN-II turned traitor.
While the Spartans and ODST's fought their way through the Array, several Covenant vessels entered the Olmarra system, having discovered the colony's location after interrogating a group of rebel fighters captured while salvaging a recent battlefield. Their chance arrival mere hours after the arrival of Battlegroup Tempest proved to push the situation into the UNSC's favour, as the Hephaestus Array was now forced to engage two foes. Though Kesson did take this opportunity to contact the rebel leader again, they refused to communicate and carried on the fight. Heavy losses were soon incurred as the Covenant's fearsome CSO-class supercarrier destroyed four more human vessels alone, though the Array's MAC guns were able to destroy two battlecruisers escorting it. Pushed into near-total retreat as they fled around Endrin, Kesson's ships were forced to rely on their assault team's success if they were to win the day.
Aboard the Hephaestus Array, the trio of Spartans were able to fight their way into the station's control room, where after dealing with a platoon of enemy troopers encountered a MJOLNIR-armoured Jack-085. Facing their former comrade in combat, they were able to outfight Jack as reinforcements took the remaining crew prisoner, though a last-ditch attack from the traitorous Spartan badly wounded Jacob-076 as he fled. Leaving him behind as the Array's remaining fighters launched a counter-attack to retake the control room, Marco and Wulf pursued Jack to a private hangar bay, chasing his Longsword fighter in a stolen Pelican all the way down to Endrin's surface, where Jack was shot down by UNSC fighters. Meanwhile, Amy-133 boarded the Array and fought her way to assist Jacob, who had taken further injuries defending the control room and was unable to walk unassisted. By this point, the station was heavily damaged and had lost its ability to coordinate MAC fire, with any remaining gun stations firing independently. The two Spartans were able to gain control of the Hephaestus Array's thruster modules, directing the damaged construct on a collision course with the Covenant supercarrier to save what remained of their battlegroup.
On the ground, Wulf and Marco had tracked down Jack's crash site, and were ambushed as they investigated it. Taken by surprise, Wulf was knocked unconscious and Marco was forced into a vicious hand-to-hand fight against Jack that exhausted the two men, and would have led to the Spartan's death were it not for sudden developments in orbit. Fending off what few rebels remained by the time they stopped communicating with Battlegroup Tempest, Amy and Jacob were presumed to have died when the Hephaestus Array smashed into the enemy supercarrier, which had all but ignored it as a threat until it was too late. While this stopped the Covenant assault in its tracks and left the remaining battlecruiser as easy prey for the three surviving vessels, Rear Admiral Kesson had already perished after his flagship was ripped apart by plasma torpedoes. With its thrusters still active, what remained of the Hephaestus Array and the supercarrier plunged into Endrin's atmosphere, spilling debris across an entire continent.
The Array's sudden destruction and subsequent fall to Endrin's surface would save Marco's life, as the sight of his life's work disintegrating above him unnerved the emotionally unstable Jack and forced him to retreat. Marco and Wulf did not pursue him, and were forced to flee the area and link up with Nef-015, who informed them of the death of their comrades. As Jack had fled towards a zone that would become the primary crash site for both the supercarrier and what little remained of the Hephaestus Array, he was assumed to have died as most of the area soon became completely uninhabitable. What remained of the UNSC battlegroup soon gathered its forces above Endrin. With Captain Uriah Maxon of the UNSC Bright New Day taking temporary command and fearing a reprisal from a bigger Covenant force, they soon abandoned the now-deserted colony world and left the Olmarra system soon after.
With so many ships lost over a completely insignificant colony world, few regarded the UNSC victory over Endrin to be a significant one. While the existence of the Hephaestus Array as a possible 'Insurrectionist superweapon' was discussed, most dismissed it as a half-functioning construct built by delusional rebels that would not have been able to stand up to a well-organised fleet even if it had been completed. While the death of Rear Admiral Kesson and a number of good officers was a tragedy, losing two extremely valuable SPARTAN-II's in the line of duty - a first for Sigma Team - was seen as a terrible sign that the war was going poorly for the UNSC.
While the Spartans present at the battle had identified Jack-085 as the rebel leader, they neglected to mention this in any reports afterwards and assured their superiors that the rebel leader had died after his Longsword was shot down. Their reasons doing so were personal, sharing the truth only with their Spartan brethren. In reality, Jack had survived the battle and his aimless dash away from Marco, only to be grievously injured and partly dismembered by burning wreckage after falling into an abandoned mineshaft. The rogue Spartan would survive this ordeal thanks to the intervention of the United Rebel Front's Omega Group, who had been operating from a bunker network on Endrin on the orders of Jack's superiors within the URF. These scientists, led by Simon Petrovich, would resuscitate and rebuild most of his body, replacing a great deal of his damaged parts with cybernetic replacements before unleashing him on the colonies some time later.