What are Hollows?

Hollows are a race of creatures which are born from Human souls who, for various reasons, do not cross over to Soul Society after their death and stay in the Human World for too long. They are corrupt spirits with supernatural powers that devour the souls of both living and deceased humans. Hollows are the principal antagonists at the beginning of the story and remain a primary threat beyond that point. Hollows settle in Hueco Mundo (Spanish, lit. Hollow World; grammatically, the phrase would be Mundo Hueco, meaning "a world that is hollow"), but can cross over to the Human World and Soul Society.