"In my world, when a person dies, they ideally move onto the afterlife on their own. If they don't, and a soul reaper does not find them to help them in time, they can...I guess corrupt is the best word for it." Ichigo tried his best to give a simplified explanation. "They lose their heart and become monsters that hunt other lingering souls and people with high levels of spiritual power. Ones that get really powerful can return to a more human-like look. You might have seen the guy with blue hair and a bone animal jaw on his face? He's a hollow. A very, very strong one."