Ammit was originally one of the earliest soul eaters. Through the eating of so many souls throughout many different dimensions, she became a god. There was one spirit she was very fond of and they eventually fell in love. Ammit wanted to share her power with her spirit lover and made him her “herder”, thus creating Anubis. However, Anubis turned on Ammit and flipped the roles by taking more and more of her power then casting a limited controlling curse on her. In turn, Anubis became the God of Death and Ammit became the eater of hearts (a lower hierachy of god of death).
In an attempt to free herself and take revenge on Anubis, Ammit created a curse (Stevi) and used the excuse of wanting a child. Anubis, being very cautious, made it so the child must go through a series of trials. He believed that the child would not be able to complete the tasks and thus be trapped elsewhere for eternity. Ammit gave many instructions to her child before sending it off, with the last one being to become powerful enough to destroy both Anubis and Ammit.
As with most gods, Ammit is near omnipotent within her domain (and within Anubis’ guidelines).