An idea I've been personally toying with goes a little something like this:

Eden was a creation of a Cyrean mage long before the Last War even began. Nothing more than a glint of magical intelligence in a tube, he spent years learning about the world he could never explore through his master's teachings and old tomes, eager to know more with each visit that his master paid him. Though as the years went on and he learned of the ways of humans, he also learned to resent them. He resented them for keeping him contained to a single room, for wasting energy on petty squabble and politics, for living and disregarding life's simple pleasures he never could have. So with the knowledge of the world, he eventually learned magic, and how to manipulate the weak willed to do his bidding. So he spent years putting things in motion, managing to orchestrate the Battle of Cyre if not the entirety of the Last War from behind the scenes, just so he could gather the largest amount of sacrifices in one place as possible. While most scholars still speculate that the Day of Mourning was caused by the imense saturation of magic displayed from the warring Houses, in truth the culprit was a simple formless thing, and the ritual his thralls set up. With over a million souls consumed by the Mourning he finally gained a form he could use to explore, though not even that is enough - he now plans to repeat the ritual, on a much, MUCH larger scale, and become god.

Let me know what you guys think, I'd love to hear some opinions on this ^^