Many hundreds of years ago, human souls with notable reiatsu began appearing due to humans and Aspects procreating. The offspring of these "halfspects", originally sent to The Aether, began to get sent simply to the Soul Society due to the Aspect in their blood getting to diluted to justify bringing them into the Aether. Four of these powerful humans decided to try and organize other powered humans, and make a deal with the Aspects to help in psychopomp duties. This deal was the precursor to the First Aspect Treaty, though the four 'Shinigami' wanted to be partners with the Aspects, instead of getting rid of them.