The Azathoth Theory/Dreaming Titans Theory
OK, so lets get some context out of the way first.
In the lovecraft mythos, arguably the highest god is one known as Azathoth. Basically, Azathoth is the root of all of existance. The concept is explained in the Dream Cycle series of short stories, that being that all of existance exists within Azathoth's dream, with everything being figments of his imagination within said dream, from other old ones to squirrels. When he wakes, everything simply ceases to exist, and when he again falls asleep a new reality is born.
It has always been weird to me that in the roaring tale, darkners get turned to stone once the roaring starts. Shouldn't they be empowered, if the titans really do come from the dark fountains? But here's where my theory comes in.
I think that the darkners are to the titans as existance is to Azathoth. The dark fountains are simply tapping into the titan's dreams, bringing life to the inanimate and creating whole dark worlds from their imagination. But like a tree dying from too many sap taps, if there are too many dark fountains at once that can wake the titans, leading to the dreams ceasing to exist and sparking The Roaring.
This would have strong themathic ties to the fact that the origin of the whole game is that it came to toby in a dream.
TL;DR The dark worlds come from the Titan's Dreams.