Ammit is a monster summoned by Shadi during his Shadow Games. In Egyptian mythology, Ammit was the personification of divine retribution for all the wrongs one had committed in life.
Shadi transformed Curator Kanekura's chair into Ammit, during a Weighing of the HeartShadow Game. While this was only an illusion, the real Ammit was let into his soul room. After Kanekura continued to lie, Ammit passed judgment on his soul, devouring his soul and killing him (in the anime, driving him insane).
Shadi also used Ammit in his Shadow Game against Dark Yugi. It held Yugi, while Yugi was asked a riddle. If Yugi couldn't answer in time, he would be left victim to Ammit. Dark Yugi answers the riddle and is released from Ammit.