I am currently planning a campaign involving primordials. What I've gotten so far is that long after the Primordial war, creation has been humming right along, up until a point where a more secretive race(using Githzerai and Githyanki stat blocks for them) began to worship a trapped primordial. Jumping to the future, the said primordial has been set free, and has freed his fellows. With the combined might of their new worshippers and their own anger at the gods having imprisioned them they have begun a new war. They are winning...
As it stands, with the gods forseeing their own failure, they attempted to protect their creations. Having nothing of this, the primordials attacked during the gods ritual and several of the gods were killed. The ritual being cut short, worked, but only on a portion of the world they first tried it upon. The gods that survived, fled to other worlds they have created, always fighting a losing battle.
This is several hundred or even a thousand years ago, and now the world, under Primordial control, is seen as nothing more than a novelty. One ancient primordial is buried deep within the worlds crust, waiting and watching, for the godly barrier to fall.
Meanwhile the races outside the barrier are enslaved, few fight back against their oppression, and look to Godshome, as they now refer to the barrier, as the only safe place within the world. The rest of the world is ruled by the primordial worshippers, titans, giants, and lesser primordial powers.
On the inside of the barrier, alot has been forgotten due to the gods interference. Seeking to give their creation a peaceful existance, all information concerning the gods and primordials war and otherworldly outside affairs was going to be wiped from their collective conscience. Due to the incomplete ritual however, the people now only believe that the gods have died, and that in their death a massive cataclysm has engulfed their entire world, and that they are all that remains. To them, they see no barrier, and to travel outside of one side of it is to travel back into the opposing side. This is the world the PC's will be starting in, Godshome(though I'll likely rename it).
Now within Godshome, not long after the gods death, fearing what may happen to them should whatever killed their gods still be alive, people of every race, including the more-so-evil ones came together. Following an Avatars fragmented memory, the people invoked an ancient ritual meant to resurrect a dead god. The ritual itself is so powerful, that the gods separated it among the knowledge of each of the races, using the avatars memory of it as a basis, the ritual was transcribed, with each race performing its own secretive part. Killing 70% or so of its participants, the ritual failed, though none know for sure why(when first created, nothing, not even magic, could enter or exit Godshome). Still sapping its participants of their lives, the ritual continued to try to fulfill its purpose, and in-so doing did in fact create an entity trapped within Godshome, plagued with the burden of several of the gods consciences trapped within one seemingly immortal body.(This NPC, the characters will have a chance to meet, and it may help or hinder the PC's depending upon the controlling conscience.) The remaining 30% of the ritual participants were scarred, some to more degrees than others(The avatar, itself, being one of them).
Over the interveneing years the races have grown separated once again as they used to be. The Avatar still remains, controlling the largest faction of human settlements. Seeking more power over the last half century the avatar has become ever more corrupted and misled by his new counselor, a primordial worshipper that has found its way within the dome. The Avatar has begun to recreate his failed ritual, believing that it can be redone to instead bestow godly powers of the deceased gods upon itself. After a few hundred years of trickery, the Avatar has finally come to its final culmination of its plan. Seeking the knowledge of its ancient ritual, the Avatar has made a deal with more than a dozen orcish tribes. The plan...to declare war upon himself and enact ancient pacts with the other races to ensure their help as he is losing. Upon their intervention, he means to pull back and let them fight to the death, and in the aftermath utterly destroy any of his remaining enemies. Thereby making it possible for him to simply walk into their kingdoms and retrieve from them what knowledge he needs.
As it stands, here are the powers that be, within Godshome:
1. Avatar and his kingdom- The Avatar knows of what once happened concerning the gods, and knows what awaits them outside of their protective bubble. He has slowly become more and more perverted by the power that he now wields. With whisperings of uprising from the other lands within the kingdom, the Avatar will stop at nothing to increase his power. Seeking the original ritual, he believes that he may be able to pervert it and thereby imbue himself with the power of the gods. He already holds several pieces of the ritual, the others being closely guarded by the other kingdoms. Recently, he has also started to be played by one of the githzerai(primordial worshipper, one of a few that have recently found their way within Godshome, but no way back out). Worming his way into the Avatars good graces, supplying a piece of the missing ritual, he has become a collector for the Avatar. Scouring the world looking for the missing pieces and supplying them to the Avatar, being one sure-fire way of bringing down the dome over Godshome.
2. Dwarven & Elven Kingdoms- Both the dwarven and elven kingdoms have been here since time immemorable. Both have also seen the slow descent of the Avatar into his madness. The higher-up, 'royalty', of each of the races either bears or knows the whereabouts of a piece of the ritual. Few know of this in the dwarven kingdom, and a protector is chosen by the king from among one clan to know of the ritual and its power, and a power they believe will one-day destroy the world yet again. In the elven kingdom, a wielder is chosen, and the ritual is marked upon their body in an intricate arcane tapestry of tattoos. The wielder is revered among all of the elves, and to be chosen as such is seen as a great honor and responsibility.
3. Primordial Powers- As of yet, the primordial powers do not hold much sway within the dome. Using all of their power and know how, few have ever been able to make it through the dome, and of those that have, no one knows whether they survived the voyage. Those that have survived the voyage, mostly still seek their masters bidding, though there are a few within the dome that enjoy the fact that their masters have no power over them here. Over the outside world, the primordials and their followers rule it with an iron fist. Without the powers of their gods backing them, the other races fell quickly to the invaders. Though there is still resistance in many areas, the primordials have mostly left the world to their followers and a few lesser primordials to finish subjugating the rest of the world.
4. Those who sought power- Within Godshome, there has become one truly terrible problem that very few, of which, are even aware. As an inadvertant side effect of either the gods plan, the ritual, or something which the primordials have done, the dead are no longer able to move on. During the original ritual, there were those who sought power, or those who after attempting the ritual turned psychotic from the myriad of lives that they destroyed to survive themselves. Of the original participants, one of them still survives. Feeding off the life forces of others, over the years he has become strong again and learned of the undead phenomenon. Having no doubt turned psychotic himself, he now believes that he'll be able to perform the ritual himself and grant unto himself the power to change the world.
5. Myrrdin's Forces- Completely hidden, the only reason why they are still around, Myrrdin's forces are the descendants of those who participated in the original ritual. Myrrdin, himself being a keeper of one piece of the ritual, knew of the Avatar before the world changed. Knowing him to be a just and honest creation, Myrrdin has since watched the Avatars downfall into madness. Believing him to be misled by some evil influence, Myrrdin still believes that the Avatar has some part left to play for the world, whether it be good or bad. Knowing that he does not contain the power to restrain or outright kill the Avatar, Myrrdin took to running and hiding from him as he was hunted. Since that beginning time, the Avatar has signed a death sentence for anyone showing any side effects from their descendants participating in the ritual. Due to this decree, Myrrdin's followers have grown over the years. Mainly humans and halflings that are under his control, recently more dwarves and elves have sought refuge with him, having felt that they just no longer belonged with their kinfolk. No human town will now accept one of the spellscarred, knowingly, and in the rest of the world they are looked upon with pity, resentment, anger, or fear. Myrrdin himself, was an extremely young elf when he helped with the ritual, and seemingly due to its influence, is now the oldest living elf in the history of Godshome.
The PC's will all be starting in the same town on the southeastern side of the continent, near to where the orcish onslaught shall begin. Most of them will have known each other most of their lives.