The mysterious overpowers were the subject of a long-running debate on the Planescape Mailing List. Here is a summary of the most frequently-discussed points about the overpowers.

Where do we learn about overpowers? Two overpowers have been revealed in official AD&D materials and related novels. They are Ao, of the Forgotten Realms world of Toril, and the entity known as Chaos/Highgod in the Dragonlance setting of Krynn.

What can overpowers do? No defined answer. Their powers as so far revealed seem to be as regulators for the gods. Ao cast the powers out of their home planes to walk Toril, and Chaos sparked a devastating war that lead to the total withdrawl of Krynn's gods.

What do overpowers do? They police the gods, and have basic responsibility for a crystal sphere.

What don't they do? They don't have avatars if at all possible, they don't answer prayers or grant spells, and they never tell mortals what to do.

Do overpowers live on the Outer Planes? No. They seem to personify crystal spheres instead. They can, however do what they like to gods of their sphere, up to killing and resurrecting them, as far as that sphere is concerned.

What about pantheon rulers? Pantheon rulers are not overpowers. Entities like Zeus are pantheon rulers, and command all their subject gods, but are not overpowers because they act just like the other gods, in that they have realms, worshippers, and a need for faith.

What is more powerful than an overpower? In a Forgotten Realms novel, Ao is seen communing with a silvery being. This creature, dubbed a metapower, is thought to be either a representation of the DM, or a hint that there is an infinite hierarchy of such beings. Someone suggested that as Ao has a two-letter name, his superior should have a one-letter one: Q. :)

What is Death? The nature of the death-being varies from sphere to sphere and pantheon to pantheon. However, in The Factol's Manifesto, it is revealed that Oridi Malefin, High Priestess of the Dustmen, worships Death itself. We concluded, with reference to Reaper Man by Terry Pratchett, Good Omens by Pratchett and Neil Gaiman, and the Qu'ran, that this being is Azrael (also spelt Azrail (punctuation?) by some), Death's Bright Angel. All death powers and faces of the Grim Reaper are aspects/subjects of this being, who could be considered an overpower.

What about the Great Unknown and the Source? These beings are thought to be one and the same. Worshipping it is akin to belief in a philosophical religion, and it never does or says a thing. It has been suggested that the Unknown is supposed to represent God/Allah/Jehovah, but I am not sure. In any event, the Unknown is not an overpower. Consciousness of this kind transcends the whole hierarchy of powers.

What about Io? Despite having a two-letter name and being very powerful, Io the Ninefold Dragon is 'only' a pantheon leader, possibly the most powerful such being.

Who made the Multiverse? We don't know. Any number of powers claim it was them, but they can't all be right. This point remains undecided.