Mediators, also called Mechanus mediators, are the balancing forces of the Plane of Mechanus. They ensure that all things on the plane - light and darkness, good and evil, liquid and solid, life and death - maintain perfect balance and harmony, and they possess godlike powers to alter reality at will in order to do so. They can create, destroy, transform, or do anything as long as it relates to maintaining balance on Mechanus. They look like clusters of geometric shapes made of strange green crystal. There are only three of them in existence.
According to myth, the mediators were created when the mysterious Powers of Creation divided the planes into various alignments, they created the Outlands as the home of the neutral powers, who would maintain balance and order. But the neutral powers bickered and fought among themselves, so the Powers of Creation banished them, creating the Spire to ensure peace and stability in the center of the Outer Planes. Many of the powers were sent to Mechanus to live, and to make sure that Mechanus was not corrupted as the Outlands had been, the three mediators were made.
Source: Planescape Monstrous Compendium Appendix One, page 68
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