Battle of Sunaion

"Today the Covenant will fall! Sanghelios will be free!"

- Arbiter Thel 'Vadam to his forces, as he leads them into combat

The Battle of Sunaion was a major engagement of the Blooding Years fought in the city of Sunaion on Sanghelios on October 27, 2558, with the joint forces of the Swords of Sanghelios and the United Nations Space Command battling the remainder of Jul 'Mdama's reformed Covenant faction. The battle ultimately led to the complete destruction of 'Mdama's Covenant and the fall of their final bastion, Sunaion.

Prelude
"Arbiter has united the keeps and formed a new alliance—the Swords of Sanghelios. With Arbiter's victories and the death of Jul 'Mdama, the Covenant remnants grow desperate. They have began an assault on 'Vadam lands, targeting Arbiter specifically."

- Shipmistress Mahkee 'Chava

In October 2558, human smart AI Cortana and Forerunner ancilla Warden Eternal began summoning Guardian constructs throughout the galaxy to their position on Genesis to allow them to enact their plans of enforcing the Mantle of Responsibility with artificial intelligences as its "reclaimers". Meanwhile, Doctor Catherine Halsey—who had been working with Supreme Commander Jul 'Mdama's Covenant—learned of Cortana's plan while on the Forerunner planet of Kamchatka with 'Mdama. Halsey contacted the rest of the United Nations Space Command and offered her information on the Guardians and Cortana in exchange for her "rescue" from Kamchatka. In response, Spartan-IV Fireteam Osiris was dispatched to Kamchatka, where they retrieved Halsey and assassinated 'Mdama. At the same time, John-117 and the rest of Blue Team went AWOL after learning that Cortana had survived her apparent destruction and sought to find her. Cortana led them to the human Outer Colony of Meridian, where the Spartans were taken to Genesis aboard a local Guardian. Fireteam Osiris, who had been in pursuit of Blue Team following their mission at Kamchatka, witnessed their departure and returned to to discuss their next course of action.

Meanwhile, without their leader, 'Mdama's Covenant was beginning to fracture, with several soldiers even deciding to align themselves with Arbiter Thel 'Vadam's Swords of Sanghelios. While some Covenant members, such as Cha 'Kulma, believed that the Covenant needed to regroup at Hesduros, the majority of the Covenant chose to attack Sanghelios and the Swords of Sanghelios with their remaining forces rather than "suffer the dishonor of a tactical retreat". The city of Sunaion in the Csurdon Sea, which had remained neutral during the ongoing Blooding Years, was viewed by the Covenant as an optimal staging site against the Swords of Sanghelios, as the city gave the Covenant quick access to the coasts of Yermo where Arbiter Thel 'Vadam's land was located. However, the Swords of Sanghelios learned of this plan and quickly established a forward operating base on the nearby shores of the Nuusra territory. Seeking to finally eradicate the Covenant, 'Vadam traveled to the Elder Council Chamber in Nuusra to lead the Swords of Sanghelios' inevitable attack on the Covenant. However, the Covenant ended up attacking the Swords of Sanghelios' military installations across Nuusra, leading to a large battle.

On Infinity, in order to both locate Blue Team and stop Cortana, Fireteam Osiris was tasked with traveling to Sanghelios and aiding the Swords of Sanghelios in eliminating the Covenant to allow them to access the nearby Guardian to travel to Genesis. Making arrangements with the Swords of Sanghelios, Osiris was brought to Sanghelios' surface by the Swords of Sanghelios on October 27, 2558 and the Spartans promptly assisted in the battle at Nuusra. After helping the Arbiter fight off Covenant forces at the Elder Council Chamber, Osiris and the Arbiter returned to the Swords of Sanghelios' nearby encampment with Halsey and Spartan Commander Sarah Palmer. Osiris was tasked with locating a Forerunner Constructor at one of the Guardian's nearby support stations to allow Halsey to input commands into the construct to force it to activate the Guardian. The Spartans continued to aid the Swords of Sanghelios in the battle at Nuusra while they located a Constructor, helping to eliminate the remaining Covenant forces in the area at the same time, including a Kraken walker. After doing so, they returned to the encampment, and Halsey input her commands into it and the construct took off to activate the Guardian at Sunaion. The Swords of Sanghelios regrouped and the Arbiter led them into battle to eradicate the Covenant at Sunaion.

Opening actions
"Swords of Sanghelios, today Sunaion will fall, and with it, the Covenant! To me, brothers."

- Arbiter Thel 'Vadam, leading his forces into combat

Arbiter Thel 'Vadam, Fireteam Osiris, and many Sangheili Storm of the Swords of Sanghelios boarded a Type-56 Lich, which led a flight of Type-57 Phantoms and Type-54 Banshees to Sunaion. Meanwhile, the Covenant ground and air forces at the city were assaulted by numerous Swords of Sanghelios warships, including Blockade Runners, Man O' Wars, Carracks, and Brigantines. Covenant forces during the Battle of Sunaion were led by several Sangheili commanders, including General Kitun 'Arach. The Covenant heavily defended the city with numerous automated Type-56 "Shrike" Anti-Aerospace Turrets attacking the Swords of Sanghelios ships. The Arbiter's squadron narrowly avoided the Shrikes, while bombarding Covenant ground forces, before eventually deploying 'Vadam, Osiris, and two Sangheili Storm onto a platform in the city. Immediately after landing, the Spartans and Sangheili eliminated nearby Covenant forces and disabled a Shrike turret. While 'Vadam led his forces into combat throughout Sunaion, Osiris embarked to the far end of the city where the Guardian was located, agreeing to disable Shrike anti-air turrets along the way to aid the Swords of Sanghelios. As the battle raged, more of the Swords of Sanghelios' fleet—including Havoc, Onslaught, and Revolution—arrived and the flight wing leader ordered Jardam Wing, Lar Wing, and Siqtar Wing to focus their fire on Covenant air forces. Osiris engaged Sangheili, Kig-Yar, and Unggoy forces and disabled the Shrikes to allow the Swords of Sanghelios' fleet to get closer to the city.

As Osiris disabled Shrike turrets, the Swords of Sanghelios were able to provide the Spartans and their own forces with air support. To aid in their defense of Sunaion, the Covenant warships began deploying several Covenant drop pods at the city to bring more soldiers to the ground battle. Additional Covenant forces were deployed to Osiris' location via Type-57 Spirit dropships in an effort to stop the Spartans. However, the Spartans were able to battle their way past Covenant soldiers and soon discovered three Shrikes firing upon Swords of Sanghelios ships, with the power source of one of the turrets protected behind an energy shield. Nonetheless, Osiris was able to eliminate the Covenant forces guarding the anti-air turrets and promptly disable all three Shrikes. Contacting 'Vadam, Spartan Jameson Locke of Osiris informed the Arbiter that the Shrikes were disabled at their location, and the Arbiter sent several Phantom dropships to the Spartans' location to deploy his soldiers. After Osiris cleared a landing zone for the Swords of Sanghelios air forces, Phantoms soon arrived and deployed many Sangheili soldiers into battle.

The Guardian's awakening
"Hunt them to the last. Today we extinguish the Covenant's light forever!"

- Arbiter Thel 'Vadam, as the Swords of Sanghelios eliminates remaining Covenant forces

Afterwards, Shipmistress Mahkee 'Chava contacted Fireteam Osiris and informed them that too many Shrikes remained for her to ferry them to the Guardian's location. However, she provided the Spartans with an alternative route to the Guardian—a passage to Sunaion's undercity that would allow direct access to the Guardian. As the Spartans descended down an elevator to the undercity, Locke contacted 'Vadam to inform him of the concussive blasts that would be set off once the Guardian was activated and warned him to move the Swords of Sanghelios' fleet away from the city before the construct's activation so that only the Covenant's fleet would be damaged. Continuing through the undercity, Osiris continued to encounter numerous Covenant forces as they fought their way to the Guardian's position. Meanwhile, the Constructor had managed to activate the Guardian and numerous Promethean forces began arriving at Sunaion, attacking both Covenant and Swords of Sanghelios forces. In the undercity, Osiris continued forward and fought their way through a large amount of Covenant soldiers attempting to block the Spartans' path. Osiris soon encountered two Mgalekgolo at the edge of the undercity and, after defeating the Mgalekgolo, the Spartans took an elevator back into the higher parts of Sunaion, eliminating Unggoy soldiers in the elevator along the way.

Meanwhile, the Prometheans were proving to be a difficult opponent to the Swords of Sanghelios and were killing many of their soldiers near the Guardian's site. As Osiris arrived back at the main part of the city, they moved past numerous dead Covenant forces as the Swords of Sanghelios were pushing them into retreat. However, the Guardian began rising out of the water and entered its activation process, as it started releasing its concussive blasts upon the city. Osiris began sprinting towards the Guardian to make it aboard the construct before it departed, but the Guardian's concussive blasts began damaging the buildings they were moving on, forcing them to avoid falling debris and crashing Covenant ships. The Spartans soon encountered Promethean Crawlers attacking Covenant forces. After eliminating the Crawlers, Osiris encountered 'Vadam killing a Promethean Knight. Together, the Arbiter and four Spartans fought alongside each other to eliminate Promethean forces, including Crawlers, Knights, Watchers, and Soldiers. As the five held off against numerous Prometheans, the Warden Eternal arrived at Sunaion. However, after a long battle, Osiris and 'Vadam succeeded in destroying the Prometheans and the Warden's physical form at Sunaion.

With the Prometheans defeated, the Guardian began preparing its final activation procedures before departing to Genesis via slipstream space. Commandeering a D79-TC Pelican, Spartan Commander Sarah Palmer and Doctor Catherine Halsey arrived at Osiris' location and picked them up to bring them to the Guardian. As Swords of Sanghelios forces decimated the last of the Covenant air forces, a Covenant Banshee managed to damage the humans' Pelican. However, Palmer succeeded in flying the damaged dropship over the Guardian, allowing Osiris to jump onto the construct just as it departed into slipspace to Genesis. The Guardian severely damaged Sunaion from both its concussive blasts and its departure into slipspace, and parts of the city began collapsing into the water below. However, 'Vadam was successful in leading the Swords of Sanghelios in eliminating the remaining Covenant forces in the damaged city. After Cortana issued her ultimatum, she dispatched another Guardian to Sanghelios which destroyed most of the remaining Covenant fleet with its standard armament. After apparently growing bored of that, the Guardian unleashed an EMP that blacked out the entire hemisphere, but also finished off the remaining Covenant ships, dropping them in the Csurdon Sea.

Aftermath
"The Guardians sounded their mighty call, and tore the stars apart. They brought us here. The ships of believers and heretics alike were flung to the fertile ground, where the survivors engage in a glorious slaughter. Yes, to die here, in the home of the gods, is the highest honor. I cannot wait."

- Muki 'Jahma, following the battle between surviving Covenant and Swords of Sanghelios soldiers on Genesis

While the Swords of Sanghelios proved victorious at Sunaion and successfully eliminated the Covenant on Sanghelios, Fireteam Osiris was brought to Genesis alongside several Covenant and Swords of Sanghelios ships that were inadvertently transferred to the planet by the Guardian. The ships were promptly flung to the surface of Genesis once the Guardian arrived at the planet, with the survivors engaging each other in combat. Once at Genesis, Osiris succeeded in meeting up with Blue Team, where the Spartans learned of Cortana's plan to use the Guardians to enforce the Forerunner's Mantle of Responsibility. In subsequent actions against the Warden Eternal's Prometheans and the surviving Covenant forces brought to Genesis, Osiris and Blue Team were unable to stop Cortana from forming an alliance of artificial intelligence called the Created and dispatching her collected Guardians at Genesis to worlds throughout the galaxy to enforce the Mantle by disabling technology on all worlds the Guardians arrived at, including several human worlds.

Eventually, Osiris and Blue Team returned to Sanghelios, using a Pelican pulled to Genesis with a Guardian and a slipspace portal opened by 031 Exuberant Witness. Upon returning to Sanghelios, the Spartans were able to land at the Swords of Sanghelios encampment at Nuusra. The Spartans met with Arbiter 'Vadam, Spartan Palmer, and Doctor Halsey, and the group began planning their next course of action to combat Cortana and her Created. After spending the night on Sanghelios, the UNSC forces left to meet up with the which had escaped Cortana's attack and begun a series of random slipspace jumps to avoid being caught. In addition, the Spartans learned that Sanghelios had also fallen victim to the Created, having been blacked out by a Guardian after the Sangheili refused to surrender to the Created.

United Nations Space Command

 * Spartan Commander Sarah Palmer
 * Spartan Jameson Locke
 * Spartan Edward Buck
 * Spartan Olympia Vale
 * Spartan Holly Tanaka
 * Doctor Catherine Halsey

Swords of Sanghelios

 * Arbiter Thel 'Vadam
 * Shipmistress Mahkee 'Chava
 * Jacul 'Arach

Covenant

 * General Kitun 'Arach
 * Khret 'Thanta
 * Tsodon 'Sakua
 * Muki 'Jahma
 * Jeem Ribfi
 * Kit Pitlimp
 * Proon Glibwik

Prometheans

 * Warden Eternal

List of appearances

 * Halo 5: Guardians
 * Halo Mythos
 * Halo: Legacy of Onyx
 * Halo: Bad Blood