Here you can read and understand the halo franchise. It includes the story itself and the story of each important character in the game. All information here is storylined as how I understood it.
Around 10.000 years in the future, unidentyfied unmaned space vessels enter the Milky Way galaxy. The ships were full with Precursor powder, the ones mentioned before. Human scavenger crews found the powder, and they brought it to their homeworlds. Later, the powder was tested on a pet animal, known as a Pheru, wich is from Faun Hakkor. The animal haven't showed any mutation symptoms. However, the future Pheru generations were agressive, mutated with strange features like green blisters and small tentacles and for feeding, they embraced canibalism, an even stranger feature for a herbivorous animal.
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The infection has spread to humans and San'Shyuum(Or the Prophets, later in the story) because of Pheru consuming and peting. The infection was later named The Flood. Because of its fast spreading rate, humans had to leave, and glass their worlds to contain the outbreak. The orders of glassing and fleeing were given by the Lord of Admirals, Forthencho.
