Transgression
The gods created the world, and in doing so they set down the laws of reality. Everything is to be a certain way. Everything is to keep in its place. You do not, cannot, change what you are created as.
One god, the Nameless, has a teensy tiny objection to this whole conceit. Clearly the solution here is to give humans magic. And not just any humans - humans whose very nature goes against how it should be. Humans who dream.
Humans who will change the world.