Wyld
The wyld is composed of swirling clouds of unfettered possibility unbound by causality or law. The creatures who live within the myst control its shape, nature, and function by pure force of will.
They Wyld itself is divided into regions by the Kosmaera. Each region's appearance and stability is governed by the Kosmaera living within.
Creatures born of reality can venture into the wyld, but risk having their own forms overwritten by roiling possibility. Harming a creature of the wyld, within the wyld is a battle of wills; the stronger will asserts causality like a weapon, forcing the creature to bend to its whim.
When a creature of the wyld enters creation, it must take form, and it becomes bound by the rules of the Great Game.