Zillah is a third being in the cosmic order and is the neutral or mediating factor between Chaos and Order. Her role as the great arbiter of Creation roots from her parentage as the first born child of Chaos and Order.
As arbiter of Creation she is tasked with keeping it balanced and satisfying the demands of Chaos and Order. Her greatest contribution is creating the Grand Design which is the system that guides and keeps the entirety of Creation in balance. This system mandates that for each world to exist a world that mirrors it inversely must exist thus creating a balanced scale. This means that for a world where good prevails, a world where evil prevails must exist. This otherwise simple concept lead to the Creation of a rather complex network