Every culture has their own version of the “underworld” — whether it’s Hell or the Elysian Fields or Diyu. In Egyptian mythology you are given a chance at a life in the field of reeds, where you can spend the rest of eternity in peace. That is, of course, unless Ammit devours your unworthy soul.
Ammit — or Ammut or Ahemait — was the Egyptian goddess of divine retribution personified. However, she was not worshipped as a goddess. Instead, her image was thought to ward off evil. Generally depicted as a demon with the head of a crocodile, the torso of a wild cat, and the hidquarters of a hippopotamus, Ammit was called the “devourer of souls.”
She was usually found in the Halls of Ma’at to await the judgement of the deceased, though she was also shown standing beside the scales of justice. Ammit would only devour the souls of those who did not measure up on the scales, and though she was called a demon, she is not evil. In fact, the person who was accused of being unworthy was given the chance to defend themselves before being judged to eternal damnation.
Though you were safe if your heart weighed just about even with the feather on the scales, many feared Ammit’s devouring of their soul — also known as the second death. However, if you led a decent life you were spared from eternal damnation. Ammit was not an evil entity in the end but, more of a keeper of order in the underworld of Egyptian mythology and a reminder to the living to lead a good life.
For more reading on Ammit (sources):