Born from a realm that gods despise, cursed energy has always been the power that rivaled the divine power of life known as mana. Whereas mana is born from the nature of the magic realm, cursed energy is born from the darkness of the dark realm that had slipped into the magic realm.
During the first holy war, the demon race had the upper hand because, unlike humans, demons never ran out of cursed energy. They could use the power freely; the only limitation was that the power was sometimes too much to handle. So far, only primordial demons were able to use the full extent of cursed energy.
However, when the half-blood demons were born, there was an imbalance in the world of magic because there was a high possibility that they could use infinite amounts of cursed energy and mana without breaking apart.