The multiverse in which all the different worlds of the campaign compendium are all composed from three fundamental planes:
The Motive is a place of perfect balance and absolute turmoil. Two oceans of light and dark meet and annihilate each other in the storm of energy that exponentially increases in intensity towards the centre of a violent vortex where the two meet. At the singularity of the vortex the energy produced by it tears apart the fabric of the multiverse, punching a rift through which the positive and negative energy pours out.
The Material contains four realms constituting the physical elements known to us: fire, water, air, and earth. Where the four elemental planes meet rages a storm of quintessence from which the four elemental planes are continually generated at one end, and swallowed into on the other. The source of the energy driving the twister is the vortex of the Motive, drilling into and out of the Material, simultaneously perturbing the otherwise calm ocean of quintessence and creating the elements, and draining them away.
The Medium contains infinite oceans, or parallel dimensions. Each of these planes constitutes its own Astral Sea, or physical universe. The space between these planes is mediated by the Ethereal Plane, although the two are merely different states of same force or field. At a particular point in the Medium an explosion of energy and matter from the Motive and Material planes erupts in an un-ending big bang. This singularity is called Primus, and from it are constituted the unending dimensions of the multiverses. The heavens, the hells, the planes filled with raw unbridled elemental power, and those few where carbon-based life are possible.
At different moments and places, different Astral Seas will be closer to one another, and therefore exert different degrees of influence. A confluence of such planes may form a cohesive sub-multiverse, a plane of mortals, angels, fey, and demons. Others may be populated with more planes inheriting more of the energy from the positive plane, others from the negative.
It is possible, even likely, that multiple vortexes originating from the Motive punch their way through the Material plane and into the Medium, creating other new big bangs in the Medium. However, if they do, they are too far away from each other for even the light from them to reach before it is swallowed by black holes floating ravenously in the interstitial space between.