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,