Please note that this lore is not canon to Bleach and may contain plot holes if more of revealed in the manga. It is only relevant to our Role Play and it's past.
In the Beginning there was naught but rock, rubble and flames. The basic foundations of what would come to be known as planet Earth today by it's inhabitants. In time and through naturally processes water was formed on this planet, along with many essential gasses that would sustain life in the years to come. Over time single celled organisms formed, ones which would reign for millions of years before the first unicellular organisms rose up from the depths of the see.
Natural selection and evolution took it's course until life was developed to the point where it contained an energy that could be called a 'soul'. As the organism died their soul would naturally join into one of two mass which slowly grew in power. Before long these beings would become sentient and were able to see the world for what it was, their world. The first one created was deemed the Host of the realm. This Host, was known as the Soul King, he was selfish and sought to no longer see a being as power of him. However, he was powerless in destroying another pool of reiryoku that was evolved and formed into a sentient being at the same time as him. This being formed into the second Host. There were both of equal power, and could not destroy the other without destroying themselves. In essence, their destinies were entwined. The Soul King... and the Demon Lord. These two Hosts realized that other pools of reiryoku were forming, and they agreed on the notion that no other should match their power. These other pools of reiryoku were simply subject to time for their punishment, and as such were split. There were 5 of these pools of reiryoku that began to form like the Hosts, however each was split in half. These now 10 pools of reiryoku evolved and became sentient beings. When they were, the SK and Demon Lord were already in conspiring a plan, a plan to not allow anyone to gain the power they possessed. So each took five of these beings as their servants, and created their respective realms. The Soul Society and Hell. A side effect of the creation of these two realms were the creation of two sub-realms, Hueco Mundo and the World of the Living, two polar opposite worlds where the living souls and Hollows could reside. Each Host then possessed a realm and five powerful servants, the Arch-Angels for the Soul King who would become the Arch-Seraphim. And the Arch-Demons for the Demon Lord. These beings would then monitor the transition of the souls through the realm so that no power would ever become such as their own.
But there was a problem. If left for too long alone without joining one like them the souls would corrupt and become monstrous. 'Hollows' as the beings would later be called accumulated in Hueco Mundo, but would terrorize the souls who made it to the Soul Society or Hell naturally. The demons were given the task of looking over 'sinners' as they were called and the Seraphim were chosen to protect the souls. Both would venture through Hueco Mundo and revert the Hollows back to their former selves, as was the task set by their creator. Each time one was created a gem would be brought into existence with them, an object that was tied to their own soul and power.
Millenia pasted like this, and resentment began to build inside these slaves to the Soul King. It was the Seraphim who rebelled first, entitled and conceited from all the power they'd been granted. All five of them rose up and fought the soul king, only to be banished to lands unknown.. The Seraphim were not fools, they knew if they tried to fight again, they would perish. So they plotted. They plotted for thousands of years, gradually making an advancement here, a gain there.
The Demon Lord had subjugated his Arch-Demons to his cause, breeding only loyalty and submission to his cause from them. And as such, they never left him. But because of that, there was a lack in the cycle itself. There were no souls being freed from the chains of fate, and Hollows began to run rampant. To solve this problem, the Soul King took the Souls of humans and unlocked their potential so to speak. They were able to build up relativly small reserves of Reiryoku. The Demon Lord then aided the Soul King, by having his demons teach these first human souls the arts of balancing. They had no powers of their own to allow souls to cross over, but they were able to show them how to forge weapons that could do so, the Zanpaktou. These Zanpaktou could also act as conduits, for Shinigami to display their true powers, without putting extreme strain on their soul. They also taught them the art of Kidou, Shunpo and healing. The Souls they first taught these arts to, would later become the Zero Squad, five replacements for the five Seraphim. With their new methods of manipulating Reiryoku and ability to have souls pass on, the now Shinigami were able to work towards restoring the balance, while the Demons returned to their Lord. They just wanted to live in and rule over hell, managing souls their and locking themselves away.
Thousands of years passed, and the Shinigami formed themselves into the Gotei 13 Harnessing this power, they became the balancers of the world. However, they became weaker than their true potential, due to something both the Demon Lord and Soul King planned. Being a separate species, they had certain qualities that the Hosts could not ascertain. This was strife, emotion, trials and tribulations. The Shinigami had lives to live, and as such began to become weaker, in both rank and individually. Suddenly Shinigami began to come and go, individuality spawned and the Gotei became even more divided. So, to match the Arch-Demons in balance, the Soul King took the 5 strongest Shinigami and set up his own Division, the Zero Division. Though they were equal to the original Arch-Demons, they were told to only act as the Soul Kings guard, and the Shinigami were to balance the souls.
The Seraphim's PlotEdit
For the longest time they went without making any noticeable move, only ever stealing away one or two high level souls at a time to build their numbers. They were waiting, waiting for a chance to tip the balance the Soul King still maintained and revert the worlds back into their original state. They would become Hosts. All of them. And with that power they'd be able to create just and peaceful realms of their own, free of tyranny and strife.
In this same time Shinigami were created by allowing strong souls to wield Zanpaktou and teaching them Kidou. They, although much weaker than the Seraphim, took their place as balancers, carrying out the Soul King's bidding to their deaths. The Seraphim could help but take a slight bit of pity on these naive souls, and so they would choose to turn a number of them into their ranks over the years, swaying them to their cause.
The same was true for the few Arrancar who existed at that point, although they had no reason to fight against the oppressive rule of their host the Seraphim always had other means to sway those in power to their side.
They've finally built up a large enough army of followers, developed their plans and have a secure foot hold to challenge the forces of the Soul King from. Each of their troops is incredibly loyal to their cause after having some of the Arch-Seraphim's reiatsu poured into them and their mind slightly maddened by the process. Once they can defeat the balancers and Hollows who oppose them their goal of finally becoming hosts themselves will be complete and the worlds will be changed for good. That was their plan at least.
In reality the Seraphim were defeated at the apex of their plan, after merging all the realms into one so they would be able to become Hosts like the Demon Lord and Soul King. After their defeat, all the Seraphim reiatsu conjugated into Shinya Tsubasa, reiatsu which he managed to cast into hell after years of trying. What lays ahead is still unknown.