Ammut the Devourer is a primordial being of chaos who claims to predate even the Nameless Gods of Irem. The Shan'iatu sacrificed thousands of humans to her in order to fuel the Rite of Return, and she transformed these lost souls into the Shuankhsen.
According to the Lost Guild, Ammut existed prior to the Nameless Gods and controlled a primordial chaos of pure Sekhem. The Nameless Gods came and imposed the Law on this chaos, but could not defeat Ammut, only constraint her. They forced her to allow the Sekhem she consumed to return to the world, and prevented her from consuming human souls who obeyed the Law. Ammut, however, was able to corrupt the Shan'iatu by exploiting their jealousy of mortals. She gave them the idea to sacrifice Azar and to build the Nameless Empire to glorify him. When the Shan'iatu realized their powers were failing, they sought out Ammut yet again with profane sacrifices, and she gave them instructions for the Rite of Return. 
The Rite required massive amounts of sacrifice, and these sacrifices died in such a way that they would not be judged worth to enter A'aru; instead they were sent directly to Ammut, who devoured all but one of their Pillars in order to transform them into Shuankhsen. Ammut proves the Shuankhsen with Bane Affinities and Bane Relics, and can reset their Sekhem when they do her some great (and usually horrifying) service.