Talk with Gabe to leave the instance. Find Latamantis in the Land of the Dead to go to the Waking World. Go to Alfsol to get the crystal of the Weeping Wall. You must beat your own demons to recover the crystal. Once you have it, Aku will return you to the Nightmare Domain. Feed Ammit the crystal.

Show Quest Info

Introduction

You were unable to cure Ammit by feeding him evil spirits, he has eaten too many good souls. To cure him you will need to retrieve the crystal from the Weeping Wall.
Notice: Find Latamantis in the Land of the Dead to go to the Waking World. Go to Alfsol to get the crystal of the Weeping Wall. You must beat your own demons to recover the crystal. Once you have it, Aku will return you to the Nightmare Domain. Feed Ammit the crystal.

NPC Dialog

Ammit was once so good, he only ate evil souls. But
he has become so sick. There is only one other
solution I know of...the crystal of the Weeping Wall.
That crystal could cure Ammit. The Weeping Wall is
in the Waking World, you must go back there and
find Alfsol. She will ask you to defeat the demons in
your heart to retrieve the crystal, this may be more
difficult than you anticipate.

Quest Completion

Ooooooh, my stomach....