Great Schism

The Great Schism, also known as the Great Betrayal among the Sangheili, was a major conflict between the forces loyal to the Covenant and an association of separatists, which split apart between October 2552 and November 2552. During the Schism, three major factions fought for victory: humans and their Sangheili-led separatist allies; the remnants of the Covenant, led by the San 'Shyuum and the Jiralhanae; and the Flood, which took over the Covenant holy city of High Charity in an effort to assimilate the other warring factions.

Background
"Thel 'Vadamee"

Even long before the start of the Great Schism, the High Prophet of Truth had plans of replacing the Sangheili with the Jiralhanae, as he saw the latter as more willing and unquestioning pawns, while he distrusted the Sangheili, doubting their reliability and loyalty. However, he kept these schemes strictly to himself and waited until the time was right to implement them. After Truth learned the location of the human homeworld, he secretly amassed a fleet of over five hundred warships, commanded entirely by Jiralhanae, at the command-and-control station Unyielding Hierophant. In September, this station and the fleet were destroyed by a group of SPARTAN-IIs who learned of its purpose shortly before the invasion could commence. This put a temporary halt to Truth's plans.

On October 20, the High Prophet of Regret made a move Truth did not expect: having discovered a significant Forerunner artifact on Earth on his own, but unaware of the fact it was humanity's homeworld, he took a fleet of fifteen ships to Earth. Immediately after Truth learned of this, he sent the remnants of the fleet he had amassed to Earth; he gave his Jiralhanae troops orders to forcibly overtake the Sangheili in command of the remaining forces on and above Earth. When Regret retreated from Earth, his Sangheili-led ships in orbit continued to engage the UNSC, but now without a leader. Only moments after, the Jiralhanae forces within the fleet and on the ground killed any remaining Sangheili, as Truth had ordered. They continued to hold New Mombasa until the reinforcements sent by Truth arrived. As High Charity and the rest of the Covenant fleet were now on their way to the newly discovered Halo Installation 05, Truth's scheme remained secret from the rest of the Covenant. Regret, who would arrive at Delta Halo weeks before High Charity and the rest of the Covenant, would serve yet another purpose for Truth: the final stage in Truth's plan.

Conflict erupts
"The Great Schism is upon us. The unbreakable Covenant Writ of Union has been split asunder. This is the end of the Ninth, and final, Age."

- Xytan 'Jar Wattinree

After the Prophet of Regret was killed by Spartan John-117 on November 2, the two remaining Hierarchs reorganized Covenant society. Claiming that the Sangheili could no longer protect the Prophets due to Regret's death, they replaced the Sangheili with the Jiralhanae as the Prophets' protectors. The Sangheili viewed this as a violation of the principle that the Covenant had been founded upon and threatened to resign from the High Council.

However, Truth further demoted the Sangheili by giving command of their fleets to the Jiralhanae. Once enough Jiralhanae were in place, Truth instigated a pogrom by secretly ordering the assassination of many Sangheili Councilors. Truth also freed all Jiralhanae prisoners and pardoned them after release. This was perceived as an insurrection against the Covenant to the Sangheili, as well as many Mgalekgolo and Unggoy. Loyalist factions were led to believe it was a Sangheili revolt. After the assassination of the Councilors, war broke out on High Charity, with the two sides polarizing quickly. However, this was at first a debate over orthodoxy and leadership. The true split occurred when the Sangheili discovered the truth about the Halos and the Great Journey. Their disillusionment changed their goal from reforming the purity of the Covenant to actively fighting against it the following day. Around this time, Truth left the Prophet of Mercy to die at the hands of the Flood, solidifying his place as the autocrat of the Covenant.

At the front lines of the original conflict aboard High Charity, the species of the Covenant took distinct sides. Loyalists followed the Prophets and included all but a few Jiralhanae, most Kig-Yar and Yanme'e, some Unggoy, and a minority of the Mgalekgolo. Separatists forces consisted of the Sangheili, most Mgalekgolo, and many Unggoy.

Sangheili-human alliance
The shaky alliance between the separatists and their former foes, the humans, was forged entirely out of necessity. Both sides had two common enemies that wished for their extinction: the Covenant and the Flood. This alliance of circumstance was forged in the control room of Installation 05, when Sergeant Major Avery Johnson and Arbiter Thel 'Vadamee united to prevent Tartarus from activating Delta Halo and firing the Halo Array. The Arbiter persuaded many Sangheili, (though far from all of them), to ally with the humans.

On November 17, the devastated UNSC Home Fleet attempted a last-ditch assault on the Prophet of Truth's Forerunner Dreadnought, which had integrated with the artifact that Regret had discovered nearly a month prior. This attack failed and a Flood-controlled battlecruiser crashed in Voi only moments after Truth's fleet entered the portal generated by the artifact. However, the separatist Fleet of Retribution, led by Shipmaster Rtas 'Vadum, arrived in pursuit of the cruiser; as the fleet glassed the city and its environs, the Sangheili and UNSC agreed to follow Truth beyond the portal.

The Ark
The joint Sangheili-UNSC fleet arrived at the Halo Array's control center, Installation 00, on December 11. During the ensuing battle, the separatist fleet completely destroyed their Covenant counterpart, despite the latter's three-to-one numerical superiority. Separatist forces later helped SPARTAN-117 disable the shield generators protecting the installation's Citadel, in which the Prophet of Truth was hiding. The Master Chief and the Arbiter entered the Citadel and fought through what remained of Truth's personal guard. After reaching the control panel they discovered that the Hierarch had been infected by the Flood; as the Prophet slipped in and out of lucidity, 'Vadam stabbed him in the back. The Flood-controlled High Charity was destroyed shortly thereafter, effectively dissolving the Covenant.

Shipmaster 'Vadum evacuated the remaining separatist and human forces after the holy city's destruction. Seemingly, only his flagship, Shadow of Intent, remained. Any remaining Covenant forces were killed when Halo Installation 04B was activated and the Ark was severely damaged by the debris; the activation also ended the immediate threat of the Flood. The Master Chief, the Arbiter, and the AI Cortana had escaped on the frigate, which was split in half when their escape portal closed too early.

After the Human-Covenant War
Though 'Vadam sought a lasting peace with humanity, many of the Sangheili were not willing to suddenly end their war with humanity. Many Sangheili insurgents, rallied together by the Servants of the Abiding Truth, incited war against the Arbiter and his allies for collaborating with humanity. With Truth dead, High Charity destroyed, and the Ark nearly destroyed after Installation 04B's firing, some of the remaining loyalist forces tried to rebuild their tattered strength to continue their war of conquest. Even though the Jiralhanae eventually collapsed into various civil wars, they still posed enough of a threat that Sangheili commanders kept up the offensive against them for at least another six years, even as they continued to lose irreplaceable warships and the war continued with no end in sight. Jul 'Mdama, a former member of the Servants of the Abiding Truth, eventually reformed the Covenant, with the Sangheili again serving as its leadership caste.

2552
September 22: October 20: October 21st: November 2: November 2-3: November 3: November 8, 2552: November 17, 2552:
 * Installation 04 is destroyed by human forces. Its Monitor, 343 Guilty Spark, escapes to Threshold. The Fleet of Particular Justice is wiped out. The Fleet's secondary flagship, the Ascendant Justice, is captured by humans.
 * A Covenant Artifact Retrieval team led by Sesa 'Refumee is investigating the Gas Mine of Threshold when he encounters 343 Guilty Spark. Inspired by Spark's words, 'Refumee rebelled against the High Prophets of the Covenant.
 * Thel 'Vadamee, the Supreme Commander of the Fleet of Particular Justice, is tried before the High Council and sentenced to death.
 * The High Prophet of Truth overrides the council and makes the Supreme Commander an Arbiter of the Covenant. Truth then sends him to kill Sesa 'Refumee and destroy his rebellion, which they dub "Heresy".
 * Start of the Battle of Earth. The defeated High Prophet of Regret is forced to retreat to Installation 05, pursued by the.
 * Moments after Regret's withdrawal, Jiralhanae within the Covenant forces quickly kill all Sangheili and seize control of the Covenant forces on Earth.
 * The Arbiter and a Spec Ops Commander, Rtas 'Vadumee lead a team of Spec Ops forces to Threshold, where they kill Sesa 'Refumee and successfully crush his rebellion.
 * Jiralhanae-led reinforcement fleet arrives to Earth and starts the excavation of the Portal.
 * The Master Chief tracks down and kills the Prophet of Regret on Installation 05.
 * High Charity and its Defense Fleet arrives at Installation 05. Witnessing the assassination of the Prophet of Regret, the Prophet of Truth replaces the Honor Guard Sangheili with Jiralhanae. Insulted, the Sangheili Councilors threaten to resign from the High Council.
 * The Arbiter and Tartarus are sent to retrieve The Index. The Arbiter successfully takes the Index, but Tartarus tries to kill him and takes the Index from him, as he was secretly ordered to do so by the Prophet of Truth.
 * Most of the Sangheili Councilors are taken to the surface of Installation 05 where they are murdered by the Jiralhanae.
 * The Prophet of Truth transfers command of the Covenant Navy to the Jiralhanae, while Tartarus declares war on the Sangheili, and the First Battle of High Charity begins. Fighting between Sangheili and Jiralhanae forces breaks out all over High Charity. As word spreads, fighting takes place throughout the Fleet, as Sangheili and Jiralhanae controlled ships attack one another.
 * The Flood invade High Charity during the battle via Slipspace in the and furthermore through the use of Pelicans to invade the rest of the city. The High Prophet of Mercy is attacked by an Infection Form and abandoned by the Prophet of Truth.
 * Tartarus attempts to activate Halo by forcing Commander Keyes to insert the Index into the Control Room's terminal.
 * Thel 'Vadamee, with the aid of Sergeant Johnson and Rtas 'Vadumee, kills Tartarus.
 * Miranda Keyes aborts the activation of Halo, placing the entire Halo Array on standby status. 343 Guilty Spark reveals the existence of the Ark.
 * The Flood takes control of High Charity and many of its warships. Jiralhanae and Sangheili fleets remaining around the city-station continue to destroy each other.
 * The Prophet of Truth departs High Charity for Earth aboard the Forerunner Dreadnought, to join the Battle of Earth. The Jiralhanae forces there are already engaged in fighting with the UNSC. Unbeknown to the Prophet of Truth, the Master Chief is on the ship, with the intention of "Finishing the Fight".
 * The UNSC Prowler Dusk witnesses the battle between Sangheili and Jiralhanae held ships above Delta Halo, both fighting each other pausing only to escalate an immediate outbreak of the Flood from the ringworld and links up with Battlegroup Stalingrad.
 * Sangheili Major Voro 'Mantakree kills his Ship Master, Tano 'Inanraree, while he is attempting to allow the Flood to enter the ship. He then assumes command of the ship, the Incorruptible, and becomes the de facto leader of the Second Fleet of Homogeneous Clarity. He leads his new forces to Joyous Exultation, pursued by the Jiralhanae Gargantum and his ships.
 * Imperial Admiral Xytan 'Jar Wattinree plans to lure the Jiralhanae leadership to Joyous Exultation and plans an ambush for them. He deploys the newly-promoted Fleet Master Voro Nar 'Mantakree to Onyx to recover the Forerunner technologies there and prevent the UNSC forces there from claiming it themselves. A NOVA bomb captured by the Supercarrier Sublime Transcendence devastates the planet moments after Mantakree's forces leave, the survivors joining Mantakree at Onyx.
 * The Forerunner Dreadnought arrives in the Solar System, and proceeds on course towards Earth at near-relativistic speeds.
 * The Forerunner Dreadnought enters Earth's atmosphere, and John-117 bails out, landing in the jungles near Mount Kilimanjaro.
 * Battle of Voi. The Prophet of Truth uses the Forerunner Dreadnought to activate the Portal, and the Loyalist fleet follows it through the portal, marking the end of the Battle of Earth. The Flood infested Voi, only to be contained by the Sangheili fleet.

Late November through December, 2552
 * News spreads wide through Sangheili fleets, who take in some instances up arms agains their Prophet escorts and, more widely, against the Jiralhanae.
 * Battle of Installation 00.
 * All Loyalist vessels destroyed over the Ark by the Separatist Fleet.
 * The Prophet of Truth, the last Hierarch of the Covenant, is executed by Thel 'Vadam with help from the Flood. The Master Chief stops the Ark from firing the Halo Rings.
 * High Charity is destroyed by the Master Chief after he overloads the city's main reactors causing them to detonate.
 * Rtas 'Vadum evacuates all humans and Sangheili via the Shadow of Intent, back to Earth.
 * The Battle of Installation 04B. Thel 'Vadam, Master Chief and Johnson battle to the control room of Installation 04B. Guilty Spark runs rampant, killing Johnson before being subsequently destroyed by John-117. Installation 04(II)'s Sentinels turned on the Master Chief, Thel 'Vadam, and the Sangheili. The unfinished Installation 04 is activated, damaging the Ark and destroying the entire local Flood infestation including the Gravemind. These final events ended the Human-Covenant War.
 * The Master Chief, Cortana, and Thel 'Vadam escape the Ark. Only Thel 'Vadam gets back to Earth.

2553
March 3:
 * The humans and the Sangheili make a memorial near Mt. Kilimanjaro on planet Earth to commemorate those who were lost in the Battle of the Ark, while also serving as a small memorial for those lost in the war.

2557
By 2557, a Covenant remnant has been estalished, led by a faction of the Sangheili and headed by Jul 'Mdama. Significant in strength, this remnant vehemently continues and pursues the work of the former Covenant's religion; worship, discovery and possesion of Forerunner artifacts.

2559

 * The Great Schism continues as conflict between Jiralhanae and Sangheili. The San 'Shyuum have vanished from the reaches of the Sangheili, Jiralhanae and UNSC empires. While the Jiralhanae wage their own internecine wars, the Sangheili themselves are gradually losing their fleets in a war of attrition, having lost the technical expertise kept exclusively by the Prophets.
 * A Sangheili Shipmaster returns to the partially-glassed human colony of Kholo.

Trivia

 * A "schism" is a division within a religious sect or denomination. The term "Great Schism" is sometimes used to describe real-world internal religious debates, most notably the East-West Schism, which led to the establishment of the Eastern Orthodox Church, and the Western Schism, which caused a split within the Roman Catholic Church.
 * In Halo: Contact Harvest, the Prophet of Truth foreshadows the Great Schism, noting that if the Sangheili ever separated from the Covenant, the entire government would collapse.
 * According to the Bestiarum, the Unggoy were too indecisive to continue the rebellion and the true political motivation of the Mgalekgolo, if any, remained a mystery.
 * In Halo 3: ODST, it is implied that the Office of Naval Intelligence was aware of internal conflict within the Covenant from early on. During the level Tayari Plaza which takes place on October 20, Buck asks Dare about the Sangheili bodies lying around the city, as they appear to have been killed by the Jiralhanae; Dare responds that said information is classified.

List of appearances

 * Halo 2
 * Halo: Ghosts of Onyx
 * Halo: Uprising
 * Halo 3
 * Halo 3: ODST
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Return
 * Halo Legends
 * Origins
 * Halo: The Thursday War