Covenant

The Covenant is a highly religious group of aliens that worship Forerunners and their technology. They believe that by activating the Forerunner's Halo Array, they would enter the Great Journey. The Great Journey is the idea the Covenant is built after. They think that by activating the Forerunner Halo Array, they will walk a path that makes the honorable and noble subjects gods while the rest who commit heresy will be left behind.

Species:

San 'Shyuum: The leaders of the covenant, also known as "prophets". They are the highest ranking species in the covenant and, with the help of elites, managed to make the covenant as it is today. They sit in the Covenant Holy City of High Charity and command the Covenant. -click for more information-

 

Sangheili: The race known manly as "elites" these tall reptile like creatures are the most honorable race in the Covenant and were the first to join the Covenant. They are the only race in the Covenant able to wield an Energy Sword. They are usually squad leaders and as the Covenant was getting started, they were the primary military force. 

Jiralhanae: More commonly known as brutes, these ape like creatures took over the elites positions after the assassination of the Prophet of Regret. They are often seen commanding troops of Grunts, Jackals, and Skirmishers. They have a very tribal commanding system and one  higher brute in a pack that wields a Gravity Hammer and uses special armor are called a "chieftain". The head chieftain of all the brutes is Tartarus.

Unggoy:  the Unggoy are called "grunts" and work as grunts. They are the lowest rank in the Covenant and used as manual labor and infantry. Although not much of a threat when alone, in large numbers they can be very dangerous. They breath super cooled methane stored on their back, which when shot will rupture making a easy way to dispose of them.

Kig-Yar: Known as "Jackals" these birdlike creatures have excellent eyesight and serve as part of the Covenant Military. They are lightly armored carry only a portable Energy Shield for protection. They are deployed in groups of three or four and are commanded by elites and brutes. They carry light weapons such as Needler, and Plasma Pistol.

Mgalekgolo: Known as "Hunters" they are the heaviest fighting class in the covenant. Though large in size, they are made of individual Mgalekgolo worms about a meter long. They wear almost impenetrable armor and have a large Fuel Rod Gun on their right arm and a huge razor sharp shield on the other.Their status in the Covenant is unknown.

Huragok: Called  "engineers", these are biological supercomputers built by the Forerunners and really don't care about the war. They are friendly and wont hurt a human or covenant unless provoked or annoyed. They use their many tentacles to take apart and reassemble machines. They are very focused creatures and will keep working on a task even if in a war zone. If attacked they will try to flee, and if killed fall to the ground, and if you are playing halo 3 ODST, explode.