Ammit first appears in the Sentinel Comics after Ra has been defeated in his first encounter with The Ennead. After Ra wanders the desert for a time, he eventually sought out Anubis to ask for additional power to defeat the Ennead. When Anubis refused, Ammit stepped in from the shadows. She offered Ra more power in exchange for his Ka and Bâ (roughly the "spark" that differentiates the living from the dead and the essence of his personality, respectively), however his heart was too balanced, and he would need to take upon himself Burdens that would make his heart weigh more than a feather, allowing Ammit to consume it.
She offered him two tasks - Blight a field and turn a lake to fire. Both of these would burden his Heart enough for it to be possible for Ammit to consume it after his death. Ra decides to try and trick Ammit. He causes the field to ripen instantly so the workers can harvest it before he blights it, and he warns the locals off the lake by causing an ox to die and rapidly decompose in the waters, turning it to poison. Thus, when he agrees to Ammit's tasks, he burns an already harvested field and sets fire to a lake of poison, cleansing it.
Ammit however, is very crafty. She knew this was something he might try. And because he had attempted to subvert a deal with a God (one of the many tenants of Egyptian Region - respect and obey the Gods) his heart was still burdened enough for Ammit's purposes. She gave him power, and he returned to defeat the Ennead.
This information is from the Sentinel Comics Universe, one of two branching universes. Products in this timeline are: Sentinel Comics RPG.
After the Ra, the Ennead, and Anubis attacked OblivAeon and were destroyed, Ammit took up Anubis role as Guardian of one of the Underworld Gates. Unlike Anubis, who steadfastly refused to allow passage between the Underworld and the real world, Ammit has her own plans. She lets some things through and others stay behind. She traps some creatures for use later. She even has possession of the Rod of Anubis. What her goals are is not obvious, but it is bound to be devious.