Human-Promethean war

The War on Requiem is an ongoing conflict that began approximately on July 20th, 2557 on the Forerunner shield world called "Requiem." The Covenant remnant, led by Shipmaster Jul 'Mdama, first engaged UNSC forces during the boarding of the derelict wreckage of the. UNSC reinforcements would subsequently arrive to fight a Covenant effort now aligned with the Didact and Requiem's native inhabitants, the Prometheans.

Background
In ancient times, the human race was once a great interstellar empire. Seeking to buy time in their war against the Flood, they attacked any Forerunner world where an infection had taken hold. Ignoring their warnings about the dangers posed by this hidden enemy, the Forerunners led by the Didact defeated humanity and forcibly devolved their civilization. But the Forerunners were weakened in process, making them easy prey to a resurgent infection.

Despite this, the Didact planned to use the Composer, a device capable of transferring organic consciousness to software format to escape the Flood's infection. But the process was flawed, and the humans who underwent the process were corrupted, becoming fit only to function as the AIs of Promethean war-frames. This, however, suited his purposes, and would let his use the Composer as both as a means of victory and for revenge against humanity. However, his wife the Librarian was horrified by his hatred, and ended up sealing him inside the shield world Requiem, which was his base of operations. The Prometheans were left there to guard his Cryptum, and Requiem remained quiet for millennia as the Forerunners disappeared.

Then, in the year 2554, the planet was found again. Seeking both a new spiritual voice for their people and a chance for revenge against the victorious UNSC, the Covenant remnant sought the Forerunner world of Requiem to unlock its secrets. Additionally, their shipmaster, Jul 'Mdama, had learned the Didact was trapped inside and sought to release him as revenge on humanity. In orbit above the planet, however, were two UNSC ships, the wreckage of the UNSC Forward Unto Dawn, and the UNSC Infinity, who had been searching for it. In the first skirmish of the war, Covenant warriors attacked the two vessel in hopes of deterring the humans' efforts.

Entering the Planet
The reawakened John and Cortana defeated the advanced scouting teams and reactivated the Dawn's weapon systems manually, destroying a CCS-class cruiser in the process.

After scanning the vessel and detecting a Reclaimer, Requiem's automated systems pulled both the Dawn and the Covenant force surrounding it through the world's outer shell and down to the planet's surface. Recovering a Warthog, Master Chief and Cortana vowed to make their way back to Earth in order to find a cure for Cortan's escalating Rampancy. They set out to find a way to contact the UNSC, circumventing the powerful jamming field surrounding the world.

Fighting through numerous Covenant hunting parties, Master Chief was attacked by advanced Forerunner defense drones which Cortana identified as Prometheans. Evading or destroying pursuit, John attempted to warn the approaching UNSC Infinity of the dangerous energy field that had disabled the Forward Unto Dawn.

After disabling two control nodes, it became clear that a great threat had been imprisoned within Requiem. The Didact, the Forerunner's greatest warrior and the individual responsible for firing the Halo array 100,000 years ago, remained in suspended animation within a Cryptum. Easily disabling John with his powerful abilities, he declared the return of the Forerunners as the galaxy's rulers and saviors. Humanity remained weak and unworthy in his eyes of inheriting the Mantle, a view rejected by his lover, the Librarian. Having always revered the Forerunners as gods, the Storm quickly swore alliance to the Didact and his armies. The Diact viewed the Covenant as mere primitives, but permitted them to serve his cause, to prevent the ascension of the human race to galactic prominence.

Narrowly escaping the collapsing command center, the Chief and Cortana returned to the planet's surface only to find their warning to the Infinity came too late. The massive warship crashed more than 70 kilometers away, with a massive force of Covenant and Prometheans closing on their position.

Evading capture, John was successful in regrouping with a group of SPARTAN-IVs led by Commander Thomas Lasky. Recognizing the Chief immediately, Lasky asked John and Cortana to clear a landing zone so that Infinity's ground forces could fall back to the ship which had fallen under siege. Commandeering a Scorpion tank, John led the attack to the crippled vessel alongside Commander Sarah Palmer's Spartans. Using the UNSC's latest weapon, the Mantis exoskeleton, John rallied a force of nearby Marines and regained control of the ship's interior. After reinitializing the ship's secondary MAC guns and missile batteries, the tide turned in the UNSC's favor. Two CCS-class cruisers and dozens of dropships were destroyed in the artillery barrage, while the Didact's Cryptum was forced to retreat.

With a momentary lull in the fighting, John was called to a strategy meeting with Andrew Del Rio, Captain of the Infinity. Skeptical of the threat of the Prometheans, Rio ordered an immediate attack on a Forerunner gravity well preventing the ship from leaving. A strike force of Spartans led by John was deployed with a Mammoth-class ultra heavy ground vehicle supporting the attack. Facing daunting odds, the UNSC was able to defeat a large armored strike force of Covenant and Promethean and disabled the gravity beam. Upon reaching his objective, John was confronted with a vision from the Librarian, who warned him of the Didact's true plans. After explaining to their history to them, she provided John with a gift, a medical treatment to counteract the Forerunner's devolution of the human genome, and a means to resist the Didact's plan to enslave the human race once again.

Regrouping aboard the Infinity, Captain Del Rio dismissed the threat of the Prometheans and prepared the ship to depart the system. Defying direct orders, John took the increasingly unstable Cortana and departed the Infinity. Armed with a Pelican gunship provided by Commander Lasky, John launched several attacks on support pillars providing the Didact's Cryptum with shield and power, with the goal of preventing him from leaving the planet and attacking Earth and other colonies.

This attempt ultimately proved unsucessful, as the Didact left Requiem aboard his vessel escorted by a flotilla of Covenant ships. Stowing away aboard a Covenant ship, Master Chief pursued the Forerunner to a UNSC research base orbiting Halo Installation 03 where the Composer was kept. Sandra Tillson, a lead researcher aboard the base, helped guide John to the artifact amidst a swarm of Covenant attackers. Despite reaching the artifact first, the Didact was successful in obtaining the Composer tearing it out of the station's hull with a gravity beam. Testing the weapon, the Composer burned the crew of the station to ash, assimilating their knowledge to power additional war machines. The Master Chief's genetic enhancements rendered him immune to the device's effects.

With the Composer in the Didact's possession, the Forerunner was poised to deal a deathblow to the UNSC, and by extension humanity. Racing back to Earth aboard a Broadsword fighter, the Master Chief was surprised to find the Infinity and a UNSC fleet waiting for him. Flying below the Forerunner vessel's shields, John was able to destroy the ship's point defense guns, giving Infinity the chance to bombard the Didact. Entering through a breach in the armor, John fought his way to the Forerunner's location.

Despite their best efforts, the Didact began to fire on Earth with the Composer, annihilating a major population center within North America. Armed with a portable nuclear device, John confronted the Didact again. While John was unable to hurt the Forerunner with conventional attacks, Cortana, nearing the end of her life, disabled the Didact's armor and immobilized him with hard light shackles. With the last of his strength, John used a Promethean grenade to destroy the Forerunner's armor and detonated the nuclear bomb manually.

Using the last of her power, Cortana was able to teleport John to a safe distance from the vessel, where he was recovered by the UNSC. Back on Earth, John rejoined Palmer and the other Spartans, preparing for their next inevitable battle.

While their defeat at Earth proved a set back for the Promethean/Covenant alliance, it did not result in their defeat. Six months later, the Infinity battlegroup returned to Requiem with a fresh compliment of Spartans, determined to reclaim the planet at any cost. The Covenant ships in orbit around the planet were swiftly defeated and scattered, demonstrating the technological advancement of the UNSC since the end of the last war. Arriving on the planet's surface, Majestic and other SPARTAN-IV squads began to disrupt Covenant operations across Requiem and recover Forerunner artifacts for study.

List of appearances

 * The Commissioning
 * Halo 4