The Prime Plane is the material existence of the Multiverse
The Phlogiston (also known as "the Flow") is a bright, extremely combustible gas-like medium that exists throughout the infinite Prime Plane. A signature property of the substance is that it does not exist within the boundaries of a crystal sphere, to the degree that it cannot be brought into a crystal sphere by any known means up to and including the direct will of deities. The Phlogiston contains several crystal sphere floats in it very slowly bobbing up and down over time. Travel between Crystal Spheres is facilitated by the formation of "Flow rivers" — sections of the phlogiston which have a current and greatly reduce travel time. Travel through the "slow flow" (i.e. off the Flow rivers) is possible, but very dangerous.
A Crystal Sphere is a gigantic spherical shell which contains an entire planetary system. Each Crystal Sphere is in fact it's own Prime Plane in effect. Each sphere varies in size but typically they are twice the diameter of the orbit of the planet that is furthest from the sun or planet at the center of the sphere.
The surface of the sphere is called the "sphere wall" and separates the void of "Wildspace" (within the sphere) from the "Phlogiston" (that surrounds and flows outside the sphere). The sphere wall has no gravity and appears to be impossible to damage by any normal or magical means. Openings in the sphere wall called "portals" allow spelljamming ships or wildspace creatures to pass through and enter or exit from a crystal sphere. Portals can spontaneously open and close anywhere on the sphere wall. Magic can allow a portal to be located. Other magic can open a new portal or collapse an existing one. Ships or creatures passing through a portal when it closes may be cut in two.
Certain locations on, within, and above Prime share a particularly close connection to one of the planes. These locations are known as manifest zones. To an extent, manifest zones defy the normal interrelationship among the planes. The effects of a zone may wane slightly when its connected plane is remote and wax equally slightly when the plane draws closer, but the connection never vanishes entirely. Manifest zones sometimes allow passage between the planes when another plane becomes coterminous to the Material Plane. Most zones are permanent and, in most cases, have had a profound impact on the land around them and the people who live there. Smaller and less important manifest zones are located across Prime, and it is possible that new ones appear even as old ones wink out of existence. The appearance and disappearance of these zones does not seem to have any connection to the linked plane’s cosmological position relative to the Material Plane.
Many classic fiend stories involve demons or devils doing their best to get into the Prime. The real question is: why? The Lower Planes, while often inhospitable to natives of the Primes, is often perfectly suited to fiends since these planes are each individually infinite in size and fiends are well suited to their environment (they speak the native tongue and are immune to the average environmental threats, and natives don’t freak out when they see them). It can’t be an issue of new lands to conquer, or even new innocents to torture, as the Lower Planes are filled with both, and in infinite abundance. So why do powerful nasties want into the Prime? The following rules are changes to the D&D cosmology, and they clear up the role of outsiders in the affairs in the realms so that more logical and fun adventuring can be had for players. The Prime is Better Than Cancun Prime Material Planes have one unique trait in all the universe: once in a Prime, you can’t be summoned or called. For fiends, this means that they are no longer subject to the hierarchies of whatever place they hang their hat. For a fiend whose True Name is being passed around like a trading card, this is a huge thing: the Prime becomes a place where he can finally determine his own destiny, and no longer be a (potential) slave to the whims of mortals or his fiendish superiors. Fiends who are plotting coups in their own realm want to be able to get to Prime so that they are outside of the authority structure of their own race, and can lay low and build up their forces for a triumphant return to their particular Lower Plane. In this way, their superiors can’t summon them and put them to the question in order to catch wind of their plans. You Can Get Room Service The second most important aspect of the Primes is that calling spells only work from the Prime. While regular summoning spells can call certain individuals, Conjuration magic of the [calling] subschool only works while in the Prime Material Plane. This means that beings that want to abuse calling magic to build armies can only do so while in a Prime. This particular rule clears up silliness like demons binding angels and forcing them to fight in Hell, or otherwise serve, which the current rules allow. Better Service for VIPs Natives of the Primes also hold a special place in the universe: they can’t be summoned or called. This is actually a pretty big deal, since this means that natives of the Primes are the premier agents in the politics of the planes. Not only can be they summon or call natives of the planes while on the Prime, but they alone are free from the Conjuration spells that enslave and bind together the Lower Planes. In addition, the basic spells of raise dead and resurrection only function on creatures native to the Prime. Other creatures can be restored to life (with revive outsider, for example), but it’s comforting to know that absolutely any Cleric can restore one’s life if she wants to – and Prime Natives live with that