really, how did such a zivilisation perish, unless the Gods felt bored of them. i hope the series answers such questions, though that might be just me asking too much of this.
Hubris most likely, people love to talk about how we can easily kill ourselves but the real threat to humanity can be something as simple as being too advanced leading to a crisis in that because they are too advanced to handle basic building blocks of their civilization.
This is actually a threat we are starting to face in the modern day due to the overreliance of technology removing things such as actual books for digital ones and formerly blue collar jobs that were the backbone of the economic and technological growth now being handled by assembly lines and the like, if something major happened, say a solar flare or some global super computer virus knocking out all technology followed by resource wars over things like food, it's entirely possible that some technologies we currently take for granted may be lost, in a century this will be a much bigger issue as it's likely that we'll become even more reliant on technology with weaknesses.
In story context it most likely is related to the master, my guess is possibly the fact that the master is born to humans almost always and who knows, maybe humanity is the progenitor of the 6 races, it's possible that this is gonna be a story of the 6 races overthrowing oppressive humans then oppressing them, but considering the path of the story i doubt it will go that philosophical.