Precursor

The Precursor is a hostile mob that spawns in few, if any, places.

Appearance
Precursors, being a transsentient species, have a multitude of different forms. Some of the known ones, however, have come to resemble various different Flood forms.

Info
The Precursors are an ancient, god-like species that existed before the current universe was created. They are ranked at a Tier 0 by the Forerunners, being able to accelerate the evolution of life, and travel across galaxies or even possibly entire dimensions. Much of their technology and science is based on the idea of neural physics, something even a species as powerful as the Forerunners could not comprehend. They would spend billions of years travelling across various galaxies and seeding them with life. When they arrived to the Milky Way, they created new life, which would be tested for the Mantle of Responsibility. One particular race was the Forerunners, hailing from the world of Ghibalb. The Forerunners, however, failed to attain the Mantle as the Precursors wanted. The Precursors then gave the Mantle to humanity. The Forerunners later found out and rebelled against their creators, nearly driving them to extinction. While some of the remaining Precursors made their way out of the galaxy or entered a state of suspended animation, others ground themselves into a dust that would allow them to rebuild themselves later on. However, this dust became corrupted, and would fail.

It is unknown as to why the Forerunners attacked the Precursors. Some say it was unprovoked, but some speculation led others to believe the Precursors wished to eradicate the Forerunners for failing their test.

Millennia before the Human-Forerunner War, humans would find a Precursor ship filled with vials of powder, which was actually the corrupted remains of the Precursors. They unwittingly used the dust to genetically alter the Pheru, a creature that became a popular pet among both humans and San'Shyuum. At first, it seemed to improve the Pheru, but later generations would become more and more aggressive. Eventually, a third of them started to sprout tentacles and eat each other, and even infected humans as they were occasionally eaten as a delicacy. The humans, after a while, managed to eradicate the Flood, at least for a time.

The Flood would return thousands of years later, infecting various human worlds. As a result, humanity had to claim new worlds, often taking them from other species. When the humans destroyed a Flood infestation on a Forerunner world, the Forerunners took this as an act of aggression, and it started a long war between the two. The Flood disappeared once again. At one point, the ancient human commander Forthencho found a capsule on Charum Hakkor, containing a being known as the Primordial. The creature was imprisoned, and interrogated. It claimed that it was not in fact a Precursor, but an artificial being created by them. Forthencho proposed that the Primordial was in fact an older, more primitive version of a Gravemind. When the humans asked it about the Flood, the Primordial's response was so horrifying, it caused most of the humans present to later commit suicide.

Following the defeat of humanity on Charum Hakkor, the Didact was allowed access to the being, believing it to be a biological weapon created by the humans. He eventually speculated that it was in fact the last living Precursor. It later escaped when 032 Mendicant Bias test-fired Installation 07. It, Mendicant, and the ring disappeared for 43 years. Upon its return, the Primordial was taken to Requiem to be interrogated by the Didact. It was there that it told him the Flood was not a Precursor creation, but the Precursor's corrupted and perverted remains after all. It also told the Didact that humanity was meant to have the Mantle of Responsibility, and that they would be tested eventually. Angered, the Didact placed it in a reverse-stasis chamber, causing it to age rapidly until it disintegrated. The Primordial's consciousness, however, would later be transferred to the current Gravemind. The Gravemind caused 032 Mendicant Bias to defect to the Flood and discover the location of the Ark. However, Mendicant was stopped by another Forerunner A.I., Offensive Bias, and was split into fragments. One of these landed on the Ark, while another was stuck inside the Keyship on Janjur Qom.

Being somewhat related to the Precursors, the Flood was able to access and control Precursor technology, including the star roads. They used this to reach planets instantly and to stunt slipspace travel used by the Forerunners.

When the Halo array fired, the Flood, the Gravemind, and all sentient life was destroyed, only to be re-seeded later. However, some Flood forms still existed within various Forerunner installations, shield worlds, and even the Halos. At an unknown point in time, there was a Flood outbreak on Installation 05. However, the ring's monitor, 2401 Penitent Tangent, ignored the alert, and was assimilated into the Flood. After thousands of years, the Flood on the ring managed to reach the Coordinated Stage with the creation of a new Gravemind. This Gravemind retained the knowledge and memories of all previous Graveminds and possibly even the Primordial. This Gravemind would supposedly be destroyed on the Ark when Installation 04B fired on the remains of High Charity. However, the Flood itself still existed within the galaxy. The UNSC even created simulations where Spartans would fight waves of Flood.

An unknown number of Flood spores survived the firing of the incomplete Halo. When a Jiralhanae lieutenant named Voridus broke the shield surrounding High Charity, the result was a massive Flood outbreak, much of which was controlled by a Proto-Gravemind. Judging from its size and appearance, it could be speculated that it was in its late stages of development.

The Domain was in fact a Precursor creation and thus considered a living thing. Due to this, it should've been destroyed with the firing of the Halo Array. However, in Halo 5, it appears to be intact and fully operational. Either a fraction of it survived or it was rebuilt by surviving Precursors.