Nippon Bleach Hell
The myth of 'Hell' a place of torment for the wicked, often told to frighten children (and less often adults) into being good, is no myth at all. Long ago, before the advent of the Shinigami and the Hollow, all souls went to soul society upon death. The problem, however, is that multiple lifespans of evil only begot greater evil. Eventually, gangs of these evil souls were populating the streets of Soul Society, wreaking havoc and sowing discord. A prison system within Soul Society was tried at first, but it wasn't long before those same souls rose up, broke out, and waged war against the heavens. With no choices remaining, the Spirit King sealed himself away, using his power to form this dimension and create the Shinigami to take his place.
A soul can never be truly destroyed, as such a prison of some sort was the best solution. To prevent 'escape' through death and early reincarnation, Hell has special magic, which forcibly regenerates any soul 'killed' within it's boundaries. Rather than banish these souls to such agonizing torment forever, however, the Spirit King's kind heart showed through, instead sentencing the wicked souls to periods of torment before being reincarnated into the living world, and given a chance to earn redemption, with each stay in Hell longer than the last. A chance which only a rare few take, however, as time in hell has a tendency to harden one against change for the better. A problem that has given the Shinigami more grief than they could have ever anticipated.
Over the centuries, those trapped within hell, frequently referred to as Demons or 'sinners', uncovered a way to set themselves free. They were able to manipulate events such, that with every soul sent to Hell, they could weaken the hellgate by some minuscule portion. By conditioning themselves mentally and spiritually, these vile souls were able to force themselves to retain their memories upon reincarnation, and set about gaining positions of power, and promoting wars and tragedies across time and the globe. Their greatest success was, predictably, World War II, which was the beginning of the end. Once the bombs were dropped, it was only a matter of time until they broke free and could exact their revenge.
The year is 1986, the so called 'International Year of Peace' and the Shinigami have fallen complacent. With the world experiencing a lull in conflict, things are finally looking up for the Gotei 13, and the majority are enjoying their R&R. Suddenly and without warning, a swirling vortex of power erupts in the sky, trailing all the way through the Dangai Precipice and ending in the Hellgate in the rebuilt remains of Hiroshima. The battle that followed was fierce, for the demons had taken full advantage of their inability to die in hell, and had trained themselves to death thousands upon thousands of times. By the time the demons had at last been sealed, The Gotei 13 had lost all of it's captains except the Captain Commander, most of it's Lieutenants, and many, many more men.
Even worse than the losses, however, is the threat that continues to lurk over the heads of the spiritual community. The demons have escaped, it's only a matter of time until they do it again.