The Arbiter

Thel was born Thel 'Vadam to the Vadam family, rulers of the powerful State of Vadam on Sanghelios. As a young Sangheili, Thel was trained in the ways of the warrior by his relative Lak 'Vadamee.[6] He joined the Covenant military, earning him the right to use the "-ee" suffix, which is added to a Sangheilian name upon joining the military. Thel fought alongside the Covenant when they attacked Madrigal in 2528.

Soon, Thel gained the rank of Shipmaster and earned the title of Kaidon within his keep.

After being promoted to Shipmaster, Thel suffered an injury in the training ring due to a mistake. His family hushed up the incident, as well as the resultant visit by a doctor, due to Sangheili culture equating the loss of blood and medical operation by a doctor with a loss of honor. During his convalescence, Thel made an old-fashioned doarmir-fur Shipmaster's cloak, wearing it always to remind him that he could make grave mistakes if he let his guard down.

At some point between 2535 and 2552, Thel's exceptional leadership and combat prowess earned him the rank of the Fleet of Particular Justice's Supreme Commander. During his time as a Supreme Commander, Thel was responsible for over 1 billion human casualties, the loss of at least 7 human worlds, the destruction of over 123 human vessels during fleet action and the deaths of over 23,000 UNSC personnel.

Near the end of his time serving as Supreme Commander, he led the victorious Covenant forces at the Fall of Reach. As the battle ended, Thel ordered all ships within his command to follow the fleeing UNSC Pillar of Autumn. Following the Autumn's course led them to Installation 04. Commanding the Seeker of Truth, he had his ships begin evacuation of the ring upon the release of the Flood, but the parasite managed to capture a ship and infect a Minor Prophet, the Minister of Etiology. The sudden widespread evacuation led to confusion as ships became infested, and he was unable to prevent the destruction of the ring and was forced to abandon it. The abandonment saved his life, but also resulted in the capture of the newly arrived flagship, the Ascendant Justice, by John-117, and he was called before the High Council to answer for these failures.

Upon being brought to the Prophet Hierarchs of Truth and Mercy, they intervened and offered to commute the sentence if he would become the new Arbiter. Understanding that becoming the Arbiter of the Covenant amounted to an indirect death sentence, but also seeking an opportunity to redeem his name to retain honor for his continuing bloodline, the Sangheili accepted the offer.

After later being betrayed by the Covenant and the Jiralhanae chieftain Tartarus, he and his Sangheili forces rebelled against the Covenant, working with humanity and the UNSC. After Tartarus' defeat on Delta Halo, 'Vadamee officially changed his surname to "'Vadam," symbolizing his resignation from the Covenant; the rest of the Sangheili followed suit.

Disgraced by the Covenant actions, the Arbiter joined forces with the UNSC to stop the Halo from activating and eradicating all life in the galaxy.

Thel, like most Sangheili, held an obsession with honor and tradition for most of his adult life. However, he was more flexible than his peers when it came to the tactics that he used, and in particular showed a keen interest in human weapons even before the Great Schism. His worldview changed greatly during the Great Schism. While once zealous and unquestioningly obedient to the Prophets' will, after learning of the true nature of the Halo rings, he became more open-minded and accepting of the humans, his former enemies. His experiences as Arbiter also made him more pragmatic than most Sangheili, being willing to accept help from the UNSC in fending off an assault on his keep.

Like the standard Sangheili combat harness, the Arbiter's armor is equipped with an energy shield. It also possesses active camouflage capabilities, but this feature can only be enabled for a limited time. Thel's primary weapons are an Energy Sword and the Vostu-pattern Carbine. The Vostu-pattern carbine served as one of the Covenant's primary medium to long-range weapons.

The Great Schism, also known as the Covenant Schism, was a major civil war between the forces loyal to the Covenant (led by the San'Shyuum and the Jiralhanae) against the Covenant separatists (the Sangheili and their followers.

Originally an ancient Sangheili order, the Swords of Sanghelios was reinstated in the aftermath of the Great Schism by the latest Arbiter and the organization's leader, Thel 'Vadam.

The Sangheili, or "Elites" as they are known by humans, served as the military leaders of the Covenant. The Sangheili are the only race within the Covenant permitted to build and command starships within the Covenant, though the crews are always composed of a mix of races. The Jiralhanae, or "Brutes" as they are known by humans, were the most recent addition to the Covenant, and served as soldiers and bodyguards within the Covenant military. The Lekgolo are a race of worm-like creatures that can combine to form large, intelligent assemblages called Mgalekgolo, known by humans as "Hunters". Clad in nearly impenetrable armor and armed with their trademark Assault Cannon, they served as the Covenant's heaviest ground troops. Hailing from Balaho, the Unggoy, or "Grunts" as they are known by humans, were the most common and low-ranking caste of the Covenant. They breathe methane gas, and thus must wear a large tank on their back full of methane to survive on other planets. The Kig-Yar, or "Jackals" or "Skirmishers" as they are known by humans, were the scouts and marksmen of the Covenant military due to their superior senses. San'Shyuum, also known as Prophets by many individuals, are a near-extinct sapient species native to Janjur Qom. Prior to the Great Schism in 2552, the San'Shyuum served as the leadership caste of the Covenant. By that time, the San'Shyuum had exerted complete control over the Covenant's religious and political affairs.

The Human-Covenant War was instigated by the High Prophets of the Covenant following a discovery which led them to believe that humans were directly related to the Forerunners, contradicting their ancient religion. Seeking to keep various political and religious orders intact, the High Prophets declared a holy war of genocide against the human species, and the discovery of their relation to the Forerunners was kept a secret. With superior firepower, numbers and technology, the Covenant would repeatedly overpower the UNSC across hundreds of human-inhabited worlds, including the heavily defended strongholds of Reach and Earth. Humanity would win only scattered and costly victories.