Deities are powerful entities who represent different magical elements.
In ancient times all the Gods worked together to shape the world, respecting each others' different ideologies and in unison they crafted what they thought to be the perfect world. They then moved on to populate the world with different races, but the God of Darkness' contributions, like the Earth's Core and its inhabitants, were looked down upon - and at one point all the Gods turned against the God of Darkness, banishing him from his own home.
In order to create balance between Light and Darkness, the God of Light chose to also banish himself. The other Gods realised that the Earth's Core served as a pillar to support the planet's surface, and it needed a God to control it. The God of Fire decided to cut off his fist and put it in the planet's center. From its blood rose the demons.