They were a combined force of alien races, and acted as the main antagonists in the Halo universe. More specifically, the Covenant's government system was a hierarchical theocratic oligarchy, underlain with a caste system, ruled by a High Council. They waged a genocidal campaign against Humanity until they were defeated due to many factors, the most crucial of which was an internal conflict. While the Covenant was dealt a series of catastrophic blows in quick succession, with most of their leadership either lost during the Flood infection of High Charity or during the Battle of Installation 00, it still continues to exist in at least some form. In exactly what capacity it continues to operate in remains unknown.
The name "Covenant" was taken from the binding pact that forged peace between the Sangheili (Elites) and San 'Shyuum (Prophets). The two species allied with each other after a long war with millions of casualties. The Sangheili-San 'Shyuum war, which devastated the two species for generations, was fought over Forerunner artifacts on the Sangheili home world, Sanghelios, with the Sangheili wishing to preserve them, and the San 'Shyuum wishing to use the artifacts. The Sangheili held a distinct tactical upper hand, as their militaristic society was ready for a war, and their warriors were among the best in the galaxy, but the San-'Shyuum had one massive advantage; an immensely powerful Forerunner warship, known as the Dreadnought. After the San 'Shyuum decided to use the Dreadnought against the Sangheili, the San 'Shyuum destroyed the Sangheili forces. The Sangheili, realizing their peril, had their warrior priests dissect Forerunner technology to strengthen their ships. Eventually they realized there was no need to fight as the main point of the conflict was now moot. When the Prophets found evidence of the Forerunners and their "Great Journey", they told the Elites of their discovery and promised to find the means of the Forerunner's transcendence. The Prophets would share this knowledge with the Elites who in turn would protect the Prophets from harm as they searched for transcendent Forerunner artifacts to activate all the Halo Installations. These ideas would be put into the Covenant's founding document, the Writ of Union. At first, the Covenant was only a loose non-aggression pact of two races held together by a common need to avoid mutually assured destruction. But, as their society advanced, so did their technology, culture, and religion. By the time of the Battle of Reach, the Covenant controlled a large part of the Orion Arm of the Milky Way Galaxy. They discovered and conquered various races. Their society had evolved into a complicated caste system, with the Prophets governing the Covenant, and the Elites put in charge of the military. The Covenant was a theocracy, based on prophecies about the Halos and The Great Journey. The primary belief of the Covenant was that activation of the Halos would allow them passage into the "Divine Beyond", where they might join the Forerunners as gods, while those who did not believe in the Great Journey would be left behind to die by the power of the Halo Rings.
The Covenant is governed by the High Council, which is led by a trio of Prophets known as the Hierarchs, and consists of lesser Prophets, Legates, and very high ranking Elites, which are known as Councilors. However, near the start of the Covenant Civil War, the Prophets began to replace the Elites with Brutes in various positions within Covenant society, including the protection of the Hierarchs and command of the Covenant Fleets. The Prophets of Truth and Mercy used the Prophet of Regret's assassination at the hands of Master Chief as an excuse to replace the Elite guard with Brutes. The Prophets then secretly ordered the Brutes to commit genocide against the Elites, thus leading to the beginning of the civil war.
Numerous Ministries exist within the Covenant government as well, each of which possesses a single role.
The Covenant Government suffered a blow in November 2552 after the Battle of Installation 05. The Covenant split into 2 different fractions, the Separatists led by Thel 'Vadam and the Loyalists that continued to follow the Prophets led soely by the Prophet of Truth after the deaths of the Prophets of Mercy and Regret.
The Covenant society is based around a caste system of the species below. Many of the alien species conquered by the Covenant are conscripted into, or gradually become part of the alliance. However, during the first contact with the Humans, The Oracle on board the Forerunner Dreadnought revealed to Truth and Regret that Humans are "My Makers", as shown by its luminary. Since the entire Covenant was based on the belief that the Forerunners had transcended, Truth and Regret knew if any of the other races found out what the Oracle told them, the Covenant would break apart.
Hailing from Sanghelios, Sangheili, or Elites, served as the military leaders of the Covenant before the Great Schism in 2552. While Prophets often had the final say, it was the Elites who organized military campaigns and naval engagements. Ship Masters commanded ships, while Fleet Masters commanded fleets, and Field Masters commanded the vast armies and the higher commanders such as a supreme commander commanded the larger fleets and armadas. Elites maintained the military structure of the Covenant and ensured the Prophets' orders were followed, as well as their own. They also had a diverse rank structure which was based off how many foes an Elite had killed. Elites also wore different colored armor to signify their rank within the Covenant. They were a major component of the Covenant and were the only caste able to maintain the military hierarchy as well as having a voice within the High Council. Later on the Elites would be tested and shoved aside from their zealous role as Covenant military leaders by the Brutes. As the Prophets and Brutes attempted genocide of the Elites, the angered Elites left the Covenant, causing a civil war within the Covenant which would end with the Covenant Loyalists adding them to their list of enemies, and the Elites forming the Covenant Separatists along with many of the Grunts and Hunters. The Separatists then became allies with the UNSC in their fight against the Covenant. They are called "Sangheili" in the Covenant language. An "ee" at the end of an Elite name, such as "'Zamamee or 'Vadumee", would signify that that Elite is a part or was part of the Covenant military, although, after the Great Schism most, if not all, Elites removed the "ee" from their names to show that they had left the Covenant.
Mgalekgolo, or Hunters, are thought to be giant monsters, but they are actually an assemblage of worm-like entities called "Lekgolo" that create a communal, armor plated form. In this form, they are known as "Mgalekgolo". They always fight and travel in pairs known as "Bond Brothers" (which technically means that not all of the colonies of worms were able to fit within one set of armor), and are the strongest of the Covenant fighting units. They wield powerful assault cannons and a massive shield resistant to any small arms fire which may also be used as a melee weapon due to its sharp sides and the Hunters' great strength. This is why if you are close enough to a Hunter, they will run at you, and swing their shield at you, which can cause major damage, but this can easily be dodged, since Hunters are relatively slow. They are usually used as shock infantry, and are only deployed on the battle field when in need of a heavy force. Their political and religious motivations are completely unknown. Their weaknesses are their exposed orange flesh, which is located on their backs, stomach, and neck. the leadership offered more protection by covering their stomachs with more armor. Their main weakness is when their back is turned, because of the lack of armor they have there. They are one of the most powerful Covenant foes, challenged only by the Elites and the Brutes.
Due to their superior senses, the Kig-Yar, or Jackals/Skirmishers are the scouts and marksmen of the Covenant military. They are about 5'6" tall and due to their lack of physical durability, they carry a portable energy shield generator during combat. Jackals are employed mainly as sharpshooters with their excellent vision and dexterity, and are also deployed to defend strategic areas. When a Jackal is under fire, it will cover its head with its hands and trot away to another position from which to fire from. The first Human contact was on a Human freighter as Jackal pirates tried to board the ship. Jackals have never been seen using grenades. This is probably because they wield both their weapon and their shield, and so cannot throw grenades while dual-wielding.
Hailing from Balaho, Unggoy, known as Grunts among Humans, are both the most common and lowest-placed caste of the Covenant, as the name implies. Despite their notable cowardice and ineptitude, they are quite dangerous in large numbers, which may have been why an Arbiter was needed to quell the Grunt Rebellion. They breathe a methane mixture and thus must wear a large tank on their back full of the methane to survive on other planets.