The voice uses "We" instead of "I," which leads me to believe that either there's more than one malicious force around, OR it's the Cloud of Darkness (who is known to refer to herself as "we, us, our" and such in other games). The World of Darkness is most likely the actual World of Darkness, as I doubt Eden is that powerful to create a completely new world and there's always a chance the Cloud of Darkness was responsible for the Idol of Darkness. What if Cloud of Darkness merges with Gaia... or with Eden? or what if Eden is the final boss drawing out memories? She is the first Sin Eater and we have been letting her summon Primals based on our questionable memory.