They initially attempted to create a race that were designed to assimilate every other race into a massive cybernetic collective, which they called the 'Cyborg' race. This plan did not entirely work, however, as the race that they created using specimens taken from all over the Galaxy decided that they were superior to the race that created them, and they broke free of the Precursor Laboratories and became the Galactic Scourge that the people of the League now know as the Borg.
The Precursors therefore decided to adapt themselves, and assimilate everything in the universe into their own collective, meaning there will be no hardships and everyone will think and work together, solely for the advancement of the species. They adapted, changing their DNA structure and fusing themselves together to form Gravespeare, and he in turn created the Precursors gift to the Galaxy, the ultimate species, the cleansing flame that will burn away strife, pain, disease, famine, crime and hate. This species was... The Flood.