18 Years ago, the Soul Society was in chaos and settled down. Rukia and Ichigo were stationed in reality. Two mysterious teenagers found Byakuya Kuchiki and promised him a normal life.
Byakuya disappeared for 18 years.
Eighteen years later, when the two sent Kuchiki Byakuya back to Soul Society, they mistakenly sent him to the Hunter World, changing his original lonely life.