Precursors, true winners

These guys are the best of the best. There isn't much history about them, they were all practically killed off by the Forerunners. They made the Forerunners and Humanity, they were so advanced they could accelerate the evolution of species. Similar to the de-evolving that the Forerunners did to ancient Humanity. The Precursors were somewhat near gods, they were so ballin they thought nothing would defeat them. They could build bridges between planets and their technology was based on neural physics which says that everything in the universe is alive but beyond our comprehension. This meant that the Precursor structures were indestructible by most means except for by the halo arrays. The Precursor's became the flood, originally the powder found by humanity I mentioned was supposed to be used to resurrect them but over the years it became corrupted as the Precursors went insane. They say that the only reason the Precursors lost to the Forerunners is because they thought that their creation wouldn't rebel against them, this idea was way to outlandish for them. Their creation betrayed them so their next creation was to know only suffering so it would never go against them. If the flood was for revenge against the Forerunners they succeded because they got recked too. The later generations of Forerunners actually didn't know that their race did this to the Precursors and they were probably really suprised when they came back with the whippin stick.
http://halo.wikia.com/wiki/Precursor <- what little more there is to know

The gravemind claimed to be the last precursor.