What I have so far (which I'll add to the op)
Long ago before the universe was created, there existed only gods and goddesses. Yuan, the most powerful of the gods, created the basis of the universe and went to eternal sleep. The remaining gods worked on various bits and pieces of the universe; the fiery god Igneel made the element of fire, the caring goddess Viga bestowed life, the kind god Aquas created the element of water, the goddess Aera created the winds, the stronghearted god Terrum forged the planets, creating the element of Earth, and the beautiful goddess Asalatasiama gave food and prosperity to the world, allowing it's life to thrive. Finally, combining all these elements, the gods and godesses created magic so that the lifeforms of the unviserse can use their gifts. Hysterius the Mad God then sent the world in utter chaos, causing the other gods to turn upon him and banish him to another dimension.
Upon being sent out of the material world. Hysterius cursed the world with demons and curses, creating the demon-lord Lucifer in the process. During his time inside this dimension, he has shifted it to resemble a large labryninth where none shall escape lest they have a portal to the actual realm. This alternate dimension came to be known as 'The Never-Ending Dungeon'. It is said that he has left the key to this dimension someplace upon the material world for a party of brave adventurers to find.
Many other minor gods and goddesses exist as well, such as Amarok the Wolf God.
Millenia after the gods and goddesses created the world, human civilization grew and prosper independently from each other, advancing in both technology and magic. Many ancienct civilizations discovered the basis of magic. Many runes that come from the ancient civilization of Herania can often be found on cursed objects.