Mana, the blood of Mystras and source of all arcane magic, is a part of every natural being's soul. Even a creature with no arcane ability whatsoever has some mana as an inherent, if small, part of them. For arcane spellcasters however, the soul is a well of mana from which they draw to fuel their magic. Typically, there are subconscious safeguards that prevent an individual from draining themselves completely of mana. Even a spellcaster who exhausts their every spell for days on end cannot push themselves so hard; there is simply a mental blockage from taxing one's self so heavily. It is unknown then, how mad the first arcanophage must have been, to have drunk so deeply and greedily from their own soul.