"A new Era"
Where there is life, there will always be death. For humans, this meant that their soul left their body. Some pure and innocent, some tainted with anger and pain from before they died. This caused for some humans to pass on to the spirit world, but only those who were pure could become Soul Reapers to help the living. Others were wiped out by the hate and anger inside of them, causing them to become a masked demon known as a "Hollow". Because the two species were the complete opposite of each other, they attacked each other on site, and this would continue for many, many years.
For years, Reapers have been at war with the hollows from Hueco Mundo. Trying to completely wipe each other out, Shinigami and Hollow alike collided and went to battle whenever they met. Being un-organized, this led to a lot of senseless bloodshed without any real victor. Though the strongest survived the bloodshed, none of the two races ever got to call an area their own, other then their respective dimensions, being the Spirit world, and Hueco mundo.
The Shinigami of the Spirit World have also been to the human world, trying to save people from becoming Hollows, and recruiting them in their own Dimension, where Souls could be trained to become Shinigami to fight at the fronts, and to kill every Hollow on sight. In their quest to eliminate the Hollows, they created agents that could be placed among the humans, called the Mod souls, a soul in the form of a small pill, placed inside of a body to walk among humans.
Some of these mod souls have escaped and started living lives of their own, and live as humans, going to school, going to work and to parties. The Shinigami are after these run away spies, they will try to either take them back, or destroy them. Hollows, still considering them to be spies, will also try to slaughter them as soon as they get them in their sights. Because of this, most do not show themselves out in the open, while others walk among humans without shame, or fear of being killed.
Just as the Shinigami developed the Mod souls, the Hollow found a way to gain new powers and win the war. By smashing parts of their mask, and the taking of souls, of other Hollows or humans and Shinigami, they could give up parts of their hollow essence and become more like a Shinigami. It took a long while for the first to become what the Hollows called "Arrancar." Though their skills were closer to that of a Shinigami than that of a Hollow, they still held on to most of their previous powers, through sacrificing some of their original power they defined their own race.
What both groups did not know, was that slowly but surely, Humans started becoming more and more part of the war, starting to create powers of their own due to all the Reiatsu around them due to the war between the Hollow and the Shinigami. Some families started wielding bows of Reiatsu, while learning about the war between the Shinigami and the Hollows. Knowing that Hollows would try to feed on humans, they became a group that would protect their fellow humans from the Hollow threat. Banding together they called themselves, Quincy. However, due to the way the Quincy did things, the Shinigami were angered by their impure way of killing, and started a vicious crusade against the Quincy to stop their way of slaughtering Hollows.
While the Quincy continued fighting the Shinigami and the Hollows, their were other Humans that started figuring out powers of their own as well. Calling their power FullBring, they were able to use Reiatsu and through an item they had always held close to them that held a lot of emotional value, they were able to create a weapon they used to amuse, and protect themselves from Hollows. Though they are not truly at war with anyone, they choose to live their own lives, not caring about the war, though word has gone out about individuals that picked sides in the war, be it for the Arrancar, or the Shinigami.
One more rare gene appeared amongst the humans, these people called themselves Bounto. They were also born with Reiatsu, and were able of turning their Reiatsu together with a material, into a living doll that did as they ordered. However, they were plagued with the need to feed on life force, attacking and feeding on other humans when they got the opportunity. Because of this, the Quincy, the Shinigami, and some extremist FullBring users targeted them believing them evil and a threat. Due to this most Bouto live in secrecy.
As the Shinigami tried fighting on multiple fronts, there were also people who betrayed them from the inside. Due to a still unknown source, some Shinigami started developing a second personality, trying to turn them into Hollow from the inside out, killing the people they once loved. Calling them Vizards, the Shinigami slaughtered their turning brethren, while being slaughtered in turn. What the Shinigami did not know, was that some of the Vizards learned to control their inner hollow, and could wield them as if it were their Zanpaktou.
Giving up on explaining, the Shinigami did not want anything to do with their turned, former allies, and continued fighting and killing them. Knowing the truth themselves, the Vizards hid in the human world, trying hard not to be part of the war any longer, though they were still attacked by Shinigami as if they were traitors, and by the Hollows, considering them to be fake Hollows and a mockery to their existence.
Noticing that as individuals, they could not win against their sworn enemies, the Shinigami and Hollow began banding together, trying to form organizations, attempting to build bases of their own to use as a home, as well as a base of operations so that they may better fight their ancient enemies.
Though there were still people who chose their own sides, the war continues even to this day, growing more and more intense now that parties and organizations are being formed. As both sides grew ever stronger, they gathered forces for yet another great war, that could soon could destroy the human world because of their warfare, leaving the world in ruin...