The monster that devours the hearts of the unworthy in the Book of the Dead is Ammit.
Ammit, whose name means “Devourer of the Dead,” appears in the Egyptian judgment of the dead. Anubis weighs a deceased person’s heart against the feather of Ma’at, the principle of truth, justice, and cosmic order. If the heart is heavier, it is judged morally corrupt, and Ammit consumes it.
Ammit is not a conventional goddess of the blessed afterlife. She is a terrifying composite creature with the forequarters of a lion, the hindquarters of a hippopotamus, and the head of a crocodile—the three animals ancient Egyptians regarded as especially dangerous. Her role was to enforce the final consequences of an unfavorable judgment.
A common mix-up is to say that Ammit eats the entire person or simply “guards” the underworld. More precisely, she devours the condemned heart, preventing the deceased from reaching the blessed afterlife and bringing about a “second death.”