Genoger, The Beast of The Rolling Plains
A subset of Humans that beforehand were powerfully devout worshippers of Jaideon. They were of a distant doctrine of Jaideon that believed it was possible to become just as powerful as the god they had revealed and he would show them the way by becoming one with all of magic itself. Those of the cabal had discerned a few words of true magic, which combined with them and changed them into different beings altogether.
After this event, the order had broken apart from those that failed to reach this goal and those that became Illumians. When the ancient doctrine died out, so did the reason for the existence of the Illumian people as well to most non-Illumians.