Millions of years ago, existed a God-like race known as the Precursors. They had evolved to the point where they were able to change their physical form by manipulating the matter from which they were made of. In short, they were the most advanced species. Ever. The Precursors were responsible for giving birth to, as well as exterminating the unfit, species. In order to maintain their creations, they created a set of rules, or laws, called "The Mantle." The Precursors decided to delegate the responsibility of upholding the Mantle and decided on a species, the Forerunners. Eventually, the Forerunners fear that the Precursors might lose the responsibility of upholding the mantle and decide to rebel against their creators in hopes of preventing that. The Forerunners traversed the galaxy trying to kill off every last Precursor. While most of them died, some of them went into hiding. Some changed their form into tiny spores in order to reanimate themselves at a later date when they thought it'd be safe. In time, these spores became corrupted and would prevent the Precursors to go back to their original form. Eventually, some of these spores would be found by an ancient human civilization and would be experimented on and studied. These spores would later on infect some pets belonging to the humans. The infected pets started mutating via breeding with other infected pets. They pets had been infected to the point where their original appearance was practically unrecognizable. We find out later on that these mutated beings (remember they're "precursor" spores) are called the flood. By the time the humans realize that they SHOULD be concerned about the flood, it is too late and the flood infection starts affecting humans. The humans then try to flee from the flood and go from planet to planet trying to escape it. The flood was overwhelming and seemed unstoppable. The humans realized that the only way to eliminate the flood would be to starve it to death and so must deprive it of anything it fed on...which is sentient life. (So the flood can be compared to zombies in space where they propagate by eating people, or in this case, beings.) One day, the humans find a being claiming to be the last remaining Precursor, called the Primordial. Once they find out of his knowledge and a way to sort of communicate with him, the humans begin to question it. The Primordial answers a question about the origin of the flood, among other things. Every time the humans came across a planet infected by the flood, they would "cleanse" the planet by killing off the flood and everything it fed on. Some of these planets, we later find out, belonged to the Forerunners. Leaders of the Forerunners gather to discuss the fate of the humans, who had been destroying colonies in some planets they control with the goal of eliminating the flood. Eventually, the humans find themselves fighting to wars at once, one with the Forerunners, and one against the flood. While eliminating the flood took priority for the humans, the Forerunners thought they should instead try and eliminate the humans. Eventually the Forerunners win the war against the humans and as punishment, devolved the entire species from a space civilization back to cavemen. This decision was made by the Forerunners' military leader, the Didact. The Didact was married to a lifeworker Forerunner called the "librarian." She watched over the remaining humans who had been kept under control. After defeating the humans, the Forerunners now found themselves with having to deal with the flood. The task turned out to be far more overwhelming than they expected and they too started becoming scared of the flood as the humans where. The flood didn't only feed upon the beings' body, but also gained all knowledge the beings possessed. The harvested information and knowledge was then held by a form of the flood known as a Gravemind. See the flood was sort of like a super organism and the Graveminds were what controlled the smaller forms. It is later found out that The Primordial is actually possibly the first Gravemind. After the forerunners won the war against the humans, they begin to also question the Primordial. They choose an AI to question the Primordial and the conversation they have lasts 47 years.

*** I'm typing this on my ipad and I have to leave, I will edit this later and finish the rest when I come back. This is the longest paragraph in the United States...