Ammit the Devourer is a demon that lives in the Hall of Judgement. Upon someone's death, if their heart does not equal the Feather of Truth in weight, (indicating a wicked life) Ammit literally devours the heart of the individual. The soul that gets eaten by Ammit is doomed for eternity, unless Osiris re-evaluates the soul and decides to bring it back using the sheut.
The dead have their hearts weighed against the Feather of Truth; if the heart weighs more than the Feather, the deceased is deemed to be wicked. The wicked heart is then thrown to Ammit, who eats the heart, banishing the soul for all of eternity. After Osiris was imprisoned, the Hall of Judgment fell into disrepair while Anubis was helpless to stop it. With nothing to eat, Ammit fell into a deep slumber.
The myths describe Ammit as female; in the series Ammit is described as small, yet depictions of the soul eater in art and on tombs show a large man-sized creature. She was said to travel to earth with blackened wings, and decapitate all whom didn't obey Anubis's orders of sacrifice.
Ammit is sleeping when Carter Kane, Sadie Kane, and Khufu are led to the Hall of Judgement by Anubis. She whimpers in her sleep when she hears her name, making Sadie wonder if she was dreaming of chasing rabbits. When Osiris returns to the Hall of Judgment, Ammit returns to her duty of devouring hearts.
Carter, Sadie, Zia Rashid, and Walt Stone go to the Hall of Judgement. When they arrive, Sadie seems to remember Ammit as a nice creature, and runs toward Ammit calling: "Poochiekins!". Her father chides her for calling Ammit the Devourer by her pet name, which irritates Sadie.
Ammit has the head of a crocodile, the body of a lion, and the backside of a hippopotamus. She is the size of a poodle, and Thoth describes her as "an adorable little thing". Carter assumed that she would be bigger, whereas Sadie calls her a hippodoodle. Carter also thinks that Ammit is a female, however Sadie is sure Ammit is a male.