The Siege of Eden V was one of the longest battles in the First Galactic War. It was also the largest single engagement until the Battle of Minerva in 2599, and unveiled the Legion's top secret IMMORTAL project, which was being conducted on the planet. Though the UNSC were able to win the battle, it came at a heavy price and proved that the supposed 'rebel' forces could pose a considerable threat in a stand-up fight. This proved to be a massive defeat for the Legion and put an end to their operations in the area for good, even crippling the organisation for several years.
On July 8th, 2583, Wulf Team, accompanied by SPARTAN-A121, was sent to Sargon, the capital city of Eden V, to meet up with an ONI contact-Codename: MATADOR, who had been gathering information on Legion activities in the region. Chief Petty Officer Maynard-X112, along with Mateo-X234 and Kate-X500 were sent in plainclothes to meet with the contact, who happened to be no other than Lieutenant Jax-007, a SPARTAN-II formerly of Sigma Team. Jax was under the command of Lieutenant Commander Alphonse Harper, a senior ONI agent.
Shortly after meeting the contact, SPARTAN-X234 was sent back to the team's mobile base, a reinforced truck with advanced monitoring devices. During the meeting with Jax and Harper, the local Militia attacked the building with RPG's, killing three ONI agents and an ODST bodyguard. The survivors made a hasty retreat, moving through several building blocks, still chased by militia, until they stopped in a large apartment building to rest. While there, a militia controlled hornet discovered their location, and fired, killing two more ONI agents and severely wounding Private Michaels, an ODST. Michaels offered to stay behind to hold off Legion attackers by blowing up a staircase with explosive charges. The survivors-SPARTAN-007, SPARTAN-X112, SPARTAN-X500, Harper and ODST Sergeant Willard then made their way to the roof.
Meanwhile, Militia forces, led by Legion military advisors, had discovered Wulf Team's truck, and began to attack. They were momentarily driven off by the now-armoured SPARTAN-X234, along with SPARTAN-X090 and SPARTAN-X432. SPARTAN-A121 drove the truck out of the city, with Legion forces in hot pursuit. They were eventually forced to turn off from the man highway, and stopped on the outskirts of a nearby forest. The Spartans inside contacted Maynard-X112 several times, becoming entrenched in their position as wave after wave of Legion attacks came at them. On the roof of the apartment building, the small group were again attacked by the Hornet, which would have killed them had it not been for the intervention of a mysterious sniper. The five then took command of the Hornet, flying it up to the location of the sniper, to find that it was SPARTAN Elena-071, part of Sigma Beta Team.
As they arrived on a landing pad on the outskirts of the city, Battlegroup Fanfare, which had recently arrived insystem. A pelican was dispatched to rescue the team as the invasion of Sargon commenced. As the pelican arrived, Lieutenant Commander Harper revealed his true intentions-he was a Legion Agent. Seeing that he could not possible kill the four Spartans, he attempted to kill himself and the others using a suicide vest. Sergeant Willard tackled Harper, knocking him off the platform, where the explosive detonated, killing them both. The four surviving Spartans were airlifted out of the city, as was the rest of Wulf Team. They were taken to the Flagship and Carrier of Battlegroup Fanfare, the UNSC Bengal.
A day later, Wulf Team, under the command of Elene-071, was sent out again to rescue a platoon of ODST's that had called for backup not long after they had been dropped in. The Spartans joined other UNSC forces who were fighting a Legion army on the Onai Plains. The ODST's were pinned down inside a crashed Cruiser. It was here that Wulf Team came face to face with the Legion's secret Project: the IMMORTALs. After a brief fight, the Spartans were forced to retreat to their gunship, having extracted the survivors. thirteen IMMORTAL cyborgs out of twenty were killed as the Cruiser exploded from charges placed by the gunship. Immortal One, the prototype test subject and leader of Pack One, also seemed to gain greater independence and awareness after the explosion damaged his neural implants.
After three months of constant battle on the planet against the surprisingly large and mobile insurrectionist force, the UNSC forces were finally gaining the upper hand, driving Legion forces back. Frion, a medium sized town, was one of their last remaining strongholds, and was heavily defended. After the first month, the SPARTAN-IV Zulu Company had arrived on the planet, and was helping greatly to eliminate the rebel forces, despite some losses due to Immortal attacks. After two weeks of trench warfare outside the town, the UNSC forces led an all-out assault, with Wulf Team and several ODST platoons at the forefront.
After a Shortsword bombing run weakened Legion defences, the Spartans of Wulf Team managed to cross the no-man's land between trenches and, led by Alex-A121, decimated Legion forces there before moving on to the town. After reaching Frion, the force split into two groups, Maynard-X112 and Mateo-X234 leading a small ODST group in clearing out houses, securing frightened civilians as they did so. After returning to the street, they were ambushed and nearly wiped out had it not been for the actions of the two SPARTAN-IV's. After fighting off a Legion ambush and linking up with a tank force, the two led the charge into the town centre, where the rest of Wulf Team were fighting off several Immortals. After destroying most of them, the rest, including Immortal One, who had begun to question his leaders and his own self-control, escaped. Frion had been taken.
By early December, UNSC forces had successfully defeated the Legion across the planet, and were wiping out the last of their strongholds in the Tendi Mountains. While the bulk of their forces were engaging the last Legion army, Wulf Team, accompanied by several ODST platoons and Elena-071, were able to infiltrate the last major Legion complex, the IMMORTAL Facility, the source of the supposed 'Spartan Killers'. After the first team, led by Maynard-X112, entered the facility, the alarm was sounded and an intense firefight broke out. Leaving the ODST's to hold their ground, the Spartans fought their way through side corridors, battling several more Immortals in their attempt to capture General Robert Amenwae, the Legion's main commander on Eden V. Searching the war room, they found most of the Legion's main leaders on the planet barring Amenwae had been killed, the latter narrowly escaping from the Spartans.
As they neared the storage bay, they were confronted by the few remaining Immortals, determined to protect their leader with their lives. Though the Spartans were pinned down, they were soon saved when several other Immortals, whose minds had been 'freed' by IMMORTAL 1, attacked their brethren, allowing three of the Spartans to slip past unnoticed. Upon reaching the Legion's command room, the Spartans found Amenwae being interrogated by the IMMORTAL, who was demanding his real name. The Immortal wished Wulf Team no harm, and after a short struggle killed the General with his armour's wristblades. After conversing with the Spartans and agreeing to surrender, he was shot dead by Elena-071, who could allow no survivors from the twisted project. It was then that the remaining Legion forces began to fire a number of highly-powered nuclear missiles. The first one destroyed Medris, a densely populated area on the planet, rather than allow the UNSC to claim it. The Spartans then headed to the launch control room to stop further missile launches after this display of power.
After breaching the room, Wulf Team were unable to stop the second missile from firing, which was set to hit Sargon city, the capital. After mopping up enemy resistance, Alex-A121, who had just arrived, uploaded a virus into the Legion systems that prevented the missile doors in the base from opening, though the launches could not be stopped. The UNSC forces hurriedly exited the base, retreating to a safe distance before the missiles detonated in their silos, obliterating the base. Meanwhile, Admiral Joseph Harris, one of the UNSC's senior military commanders, had deliberately ordered his ship, the UNSC Grievous, to intercept the missile, having evacuated his entire crew. The Admiral's sacrifice, manually piloting the ship to it's doom, saved the lives of three million people in the city, effectively ending the Siege. On the ground, the remaining SPARTAN-IV's regrouped and were picked up by the UNSC Bengal.
The devastating losses inflicted here nearly crippled the Legion's operations, causing several high-ranking members to be removed or executed as a result. Back on Minerva, the Legion's home base, their leader, Roger Maxon was approached directly by a mysterious individual, who supplied him with a vast storage of information and supplies, enough to continue the war against the UNSC for many more years. This conflict confirmed that in spite of the Legion's vastly inferior resources and supplies, they were capable of fighting off UNSC forces on even footing for a short period of time, something that worried those at HIGHCOM. The full details of the IMMORTAL Program were not revealed to the public, though the UNSC reported that their foes were performing highly illegal Human experimentation procedures. This propaganda campaign would severely weaken the Legion's foothold in the Inner Colonies.