Overdeities are normally classified by theologians as deities with a Super Weight of 6 of 7, making them capable of creating and destryoing entire large-scale universes. They normally have little interest in the material planes except when events occur that would interfere with their long-term plans. These beings almost never grant spells or answer prayers. Their minds are so powerful that attempts to telepathically contact these beings usually results in brain-death or insanity.
Most of the time, there are only three known overdeities in existence: Eru, Azathoth, and Yog-Sothoth. Only Yog-Sothoth has the ability to raise other beings to the level of overdeity. Those that become new overdeities usually leave the known multiverse to work on their own creations.