Origins

In the beginning, the only type of magic was divine magic. It was an undifferentiated chaos, a primordial ooze, that gradually coalesced into the first Gods.

A massive, terrifying, incomprehensible entity from outside the Arcane Realms came and attacked the Old Gods. They killed it, creating the Abyss from its corpse.

The first Gods began to create the world, and in the process, created natural magic to run the world.

The sundering of the Queen of Monsters created the first dragons, the first mortal arcane casters; and also warped the natural magics she'd shepherded, birthing the monsters.   This arcane magic did not include the school of necromancy, which was created much later, during the curse. At this point, the line between life and death was the purview solely of divine magic.

A group of mortal arcane spellcasters warped reality and the nature of arcane magic in an epic ritual.