From what i understand, the Precursors created the Forerunners, it says so in the last page of the book. The precursors probably didnt think the forerunners would rise up against them and were most likely defenseless as it was described as "ruthlessly destroyed". The precursors probably reached a point in their existence that they didnt think they needed to control other races. After the Forerunners over-threw the Precursors they made sure not to let any species grow too strong because they didnt want what they had done to happen to them. They disguised this as The Mantle. For an unknown reason, there was one Precursor who was locked away, inside a very powerful containment wall. He is referred to as The Captive in the book. Described as having an upright stature, large, 4 arms, 2 legs, a flat face with multiple eyes and a long scorpion like tail with barbs that is based at the back of its neck. Near the end of the Precursors existence, they created a bio-weapon, the flood. It was very cleverly instigated and disguised as well, no one could have seen or predicted it. In an organic powder form, found in giant ancient ships, humanity tested the powder. It was harmless to humans and only had an effect on domestic, at the time, a positive effect . After generations of breeding with this mutagen in their genes, the first true flood was born. After fighting for many years, humanity found a cure. It would fight fire with fire. Humanity genetically programmed 1/3 of its population and essentially sacrificed them to the flood. In doing so, humanity counter infected the flood and was able to push it to the edge of the galactic arm and out of our galaxy. During this Human-Flood war, humanity was also fighting with the Forerunners, fighting a double sided war. It was no wonder we lost to the forerunners. Had it been a fair fight it might not have turned out the way it did.
Most people i hear arguing think the Captive is the gravemind because of how he looks, and while possible, i dont think that is the case. I think all precursors looked like the captive. The descriptions in the book didnt make him seem like he had been infected. After the captive was released, he could have been swallowed by the flood, escaped or even killed when the ring was fired. One thing that is sure, the Precursors must have been immensely powerful beings, both in strength and mentally. The captive was millions of years old, much older than any forerunner. Which probably means they discovered immortality and never died from old age. Whatever the case, the biggest mystery in this book isnt the flood, the forerunners or humanity, its the precursors. They may have been brought to their knees by their own creations, but they left a parting gift that would haunt the universe till the end of time.