People in most ancient civilizations were afraid of their gods. This was not true in ancient Egypt. The Egyptians loved their gods. They had little fear and great wonder. 

There was one exception - the god Ammut. Almost everyone in ancient Egypt was afraid of Ammut! Ammut was the Devourer. The ancient Egyptians believed if you did something bad, your heart would be heavy, and the god Ammut could suddenly appear and gobble you up! 

The god Ammut had a big part in the weighing of the heart ceremony. When you died, the ancient Egyptians believed you traveled to an afterlife, a heavenly place where you spent eternity. You had to earn your way. There were rules. To enter your  afterlife, you had to have a light heart. Light hearts were earned from a lifetime of doing good deeds. 

To find out if your heart qualified for the trip to the afterlife, your spirit had to enter the Hall of Maat. The god Anubis weighed your heart. The god Thoth recorded the findings. (In ancient Egypt, everything was recorded and written down.)

If your heart was light, lighter than a feather, you passed the test and entered your afterlife.

BUT, if your heart was heavy because your deeds were dreadful, the god Ammut would suddenly appear ... and eat you up!

No one wanted that, so nearly everyone in ancient Egypt did good deeds to keep their heart light.