The reality, for some difficult to comprehend, is that these enigmatic beings were once like you and me. Regular human minds trapped in regular human flesh. Or, for species that can control Magic, animal minds in animal flesh. And that is where the key to their Demonic origins lies: Magic. All Demons are creatures who have collected vast amounts of True Spells, and used them to further themselves into the Void.
For the uninitiated Mages: True Spells are Spells with only the tiniest possible imperfections in their crafting and casting. Where regular spellcasters have to be wary of faults in there Spells and their casting, Demons have to be orders of magnitude more vigilant about ruling out all modes of failure. Not only do they augment themselves with the Spells’ effects, they intend to retain them forever. Forever is a very long time during which a lot can go wrong. And if something does go wrong, forever is a long time to suffer the consequences.
Note that the augmenting Demons do to themselves is different from traditional casting of augmentation-class Spells. Where a normal user applies the effect to their target’s physique, Demons force the Spell around their very Soul. The Spell then acts as a filter. Their flesh, augmented by new power. Their mind, listening to the Soul in ever more wicked ways. This filtering isn’t limited to one direction. Their perception of the world distorts, the Magic surrounding them, all but lost when it reaches their Soul.
Over time, they add more and more layers to the nesting doll of True Spells surrounding their Soul. More and more wicked they become, but never satiated with power for they can no longer perceive it.
They have put their Soul in the fate of the True Spells they collected, curated. But, Demons are not the kind to trust. Even the truest of True Spells have the potential to fail in their purpose, to turn around on its caster, to present unexpected side-effects. And so they seal the Spells around their Soul tightly, but leave the tiniest cord unraveled.
Of course, such a weakness is securely hidden behind a Spell Lock. And for creatures as powerful as Demons, the corresponding Spell Key is long. In the depths of their wretched minds they keep it, hidden, concealed. It is their True Name. And those who can speak the True Name of a Demon can tug at their strings and slowly unravel layer upon layer of augmentations, filters, powers. At their very core, the speaker of the True Name will find a tiny, fragile Soul, evaporating into nothing before it can feel the cold air.
Such is the cruel truth of Demonkind.