The Law of Transubstantiation states that beings who are not indigenous to the material planes will require a source of power to manifest physical bodies there. Such beings include ghosts, phantoms and gods who generally dwell behind the Aetheric threshold.
The transcension from a lesser being to an entity of divine influence is made possible through ancient and fragmented relics called ossigils. Ossigils are the weightless bones of long dead gods. These remnants are still charged with the essence of the old gods and vary in potency. By acquiring an ossigil of at least mid-level resonance, a lesser spirit effectively becomes a godling. This increase in power makes transubstantiation in the material plane possible for limited periods of time. Gods in possession of many ossigils are very powerful players in the cosmic theaters of campaign worlds.