SAN'SHYUUM / PROPHETS

Planet or Origin: Janjur Qom, a now destroyed planet with a gravity lower than Earth in the Milky Way galaxy.

The San 'Shyuum are a physically frail mammal-like species acting as leader caste in the interspecies interstellar community known as the  Covenant overwhich they exert complete religious and political control by enforcing a misguided theology based on the false belief that firing a interstellar-range weapon of mass destruction left by the now extinct species known as the Forerunners and known as the Halo Array will start what they call the Great Journey, transcending them into a god-like status. They are also responsible for studying all other Forerunner artifacts - considered holy - and with them to develop new technologies. Their position of power is based on the claim of having evolved on a former Foreunner colony.

Physically frail creatures, they prefer to move around using anti-gravity technology. A life of physical inactivity leaves most if not all Prophets withered and weak but they still remain a strenght greater than that of humans in their arms.  Each Prophet has distinct, often fur-covered lobes of skin hanging underneath their chin similar to a beard, known as "wattles". Prophets of greater age have skin lobes on either side of their heads reminiscent of ears; despite this resemblance, the San 'Shyuum actually receive sound at the back of the head. Older Prophets appear to lose the pigment in their skin as they age and turn a shade of grey, then white. Prophets have three digits on each hand. They have parts of their brain showing through absences of skin on their heads, while skin is still there, it may be possible that they evolved to allow their brain to grow to such a size that the skin is there simply to hold it in place. Anagatictechnologies gives them long lifespan but the fertility cycles of their females are short and few and far between, which would make it difficult for them to have children. 

With regards to naming practices, each San 'Shyuum has a given name and a family name. However, once they rise to a certain level in the bureaucracy, they prefer to be addressed by their title.

Sometime around 110,000 B.C. the San 'Shyuum were part of an alliance with the Humans' empire, forming a major power in the Orion Arm of the galaxy. During this period, they encountered the Flood for the first time, but the spread of the infection was successfully pushed back by the humans while it was still relatively minor. This alliance ended when the San 'Shyuum surrendered to the Forerunners during the following Human-Forerunner war. All San'Shyum on Janjur Qom were killed when the Halo Array was fired during the next Flood outbreak dubbed the Forerunner-Flood War. Their homeworld would eventually be reseeded with specimens that were preserved on the Forerunner Ark structure. Having lost nearly all memory of the Forerunners, the San'Shyum  discovered Forerunner technology on their homeworld and based their religion around the artifacts they found. The beliefs they developed led to bloody civil wars and part of the population leaving their planet, leaving it deeply scarred toward 2100 BCE.  
The war between the Sangheili and the San 'Shyuum began soon after their first encounter in 938 BCE. Sangheili believed that Forerunner technology should not be touched or used for personal gain while most of the San 'Shyuum believed otherwise, and disassembled Forerunner relics to produce their own versions. Their first meeting resulted in a bloody engagement. Physically the Sangheili were far superior. One Sangheili warrior was the equal to at least ten San 'Shyuum. However, technologically, the San 'Shyuum had the advantage and wiped out the Sangheili armada using hit-and-run tactics. 
Eventually in 852 B.C.E., in order to ensure the survival of both races, the Sangheili gave up their own conviction, and the two races merged to form the early Covenant with the San 'Shyuum as the head religious leaders and the Sangheili as the physical backbone and defenders of the Prophets. Soon after, their homeworld was destroyed approx. 648 BCE as the result of a natural stellar collapse and they moved to the mobile-planetoid High Charity which was in turn destroyed by the Sangheili when it was the scene  of a new Flood outbreak in the 2550ies. Their current total population is now estimated at less than one thousand.

Source: Halo Universe, Halo Wikia

PRECURSORS

The Precursors are a non humanoid sentient species that developed an advanced spacefaring civilisation. Their civilization was based around the Mantle, a belief hat it was their role to protect all life in the galaxy. They presumably monitored the evolution of other sentient species throughout the galaxy, intervening in ways that ensured their civilizations would follow paths of peace, free of conflict. They were chased and destroyed by one of those younger species, the forerunners, in a war that took place some 150 000 years ago, presumably upon learning that they had chosen the humans as heirs to the Mantle instead of them. The Precursors unleashed the Flood to avenge this act of agression.

Precursors are believed to be"very large", with 4 upper limbs and 2 legs. Also having an "ugly head shaped like arthropods" - like spiders or crabs or scorpions - they have oval, faceted slanted eyes and a "flat face". Coming from the back of the head is a long tail with a 2 meter barb coming out of it but no visual record of individuals of the species exist as of now.

Source: Halo Universe,

FORERUNNERS / HOLY ONES / ANCIENTS / CREATORS

Planet of Origin: Ghibalb, a planet  located in the outer edges of the Orion complex. Once a beautiful, verdant paradise of a planet, it was cleansed of all life when an experiment in stellar engineering accidentally caused several nearby stars to supernova. It is now heavily irradiated and uninhabitable.

The Forerunners are a sentient species that developed an advanced FTL interstellar civilisation and praticed world building. They are responsible for construction of the 2 Arks structures that are used to ringworld-type fortress (from 10 000km  to 30 000km in diameters) that formed the Halo array around the galaxy. The Array was designed to kill all sentient life in the galaxy which was not protected inside specially designed Dyson-type shield worlds and therefore rid the Milky Way of the Flood parasitic lifeform.

While not record of the Forerunner's appearance remains, their battle armors show them to have a humanoid shape disturbingly close to the human form.

A very advanced civilization, they came to power after the destruction of another race, known to them as the Precursors. The Forerunner believed that the Precursors were responsible for their creation. Soon after, the Forerunners became the dominant species in the galaxy, their empire spanning over three million fertilized and inhabited worlds. The Forerunners reached their peak before activating the Halo Array around 100,000 years ago in the closing days of the Forerunner-Flood war.

Their civilization was based around the Mantle, a belief they inherited this belief from the Precursors that it was their role to protect all life in the galaxy. They presumably monitored the evolution of other sentient species throughout the galaxy, intervening in ways that ensured their civilizations would follow paths of peace, free of conflict. One such occasion is the devolution of the prehistoric human empire officially to prevent further conflict emanating from what they considered a "violent species" some 110 000 years ago, but officiously to prevent them from taking their rightful place as the Precursor's heirs to the Mantle. After their superiority was insured, the Forerunners reached a point of disarmament, believing they no longer needed weapons in their rule of the galaxy, which ultimately weakened them when the Flood attacked.

When the Forerunners were unable to contain the Flood outbreak, the galaxy entered a state of war.  However, all their tactics, were they to turn star novae to eradicate infected systems, proved only to slow, but never stop the expansion of the Flood. They realized that the only way to stop the Flood was to deprive it of any and all hosts, thus eliminating its potential to grow. Eventually, the Forerunners created the Halo Array, a weapon of last resort designed to starve the Flood to death by killing all sentient life with enough biomass to sustain them. 100 000 years ago, after having exhausted every alternative, they ctivated the Halo array, killing themselves and all sentient life of sufficient biomass in the Milky Way, with the exception of those species safely placed on the Ark. After the Array was fired and the Flood had been eliminated, the Forerunners left behind an automated system of automatons, much like the Sentinels, and AIs to spearhead the reseeding of the galaxy with those data logs, embryos, and specimens contained on the Ark.

The Forerunners are adored like gods by the alien league known as the Covenant. The fact that the Forerunners appeared to be genetically related to the humans explains why the Covenant seeks to exterminate humanity, knowing that they are the Forerunner's designated heirs and would undermine their power.

Source: Halo Universe,
adapted from the Halo Wikia's  Forerunners and other relevant entries.

FLOOD

The Flood is a parasitic alien life form driven by the need to infest sentient life it encounters in an effort to spread. Nearly 100 000 years ago, its spread forced the now extinct interstellar civilisation of the Forerunners to kill themselves and all other nearby sentient life in an effort to starve the Flood to death.The Flood is believed to be an engineered living weapon developed by the Precursor species as it was being eradicated by the Forerunners in a war that took place more than 150 000 years ago.

The most common form the Flood, dubbed “Infection form” can reproduce itself without the help of external biomass. It seeks to drive sharp spines in any nearby sentient lifeforme to tap into their nervous system and incapacitate them while it burrows in their body and begins the mutation process, bringing the host under Flood control. Hosts that have been recently converted are noted to be weaker than those who have been under Flood control for long periods.

Depending on the size or condition of the hosts, the Infection form turns them into into either "Combat forms" or “Carrier forms”. Combat Forms are extremely powerful and able to resist extreme punishment. They can use the host’s technology or the long, whip-like tentacles developed during their mutation to attack any non infected lifeform. The "Carrier Form" is created if a "Combat Form" is too damaged or ill-equipped to prove well in combat. Carrier forms hosts’s upper halves decome bubbley in appearence and carry around infection forms that are developing.

Once the flood’s absorption of sentient biomass has reached a certain critical level, it develops a centralized intelligence, called gravemind, from the bodies it has infected into a massive , tentacled intelligent entity resembling a large Venus Flytrap, capable of movement and linguistic communication which litteraly controls the flood. As a gravemind develops, it is able to exact more control over the lesser flood forms and is eventually able to develop entirely new forms referred to as Pure Forms. Pure forms do not require a host body to create and are instead formed from degraded biomatter. Pure forms are capable of limited shape shifting that allows them to quickly change their role within the flood.

Source: Halo 1, 2 & 3 Console games and related novels Halo: The Flood, Halo: First Strike