Mystery Sack is a Unique Item which Emperor Tengu will give the player when he sends them on a journey to kill the notorious Grievewrath. The sack contains Tengu's royal seal so that the character can retrieve the "Elixer of Godly Powers" from Clunknad Spanner, the skeleton wizard.

Item Description
Beware of spoilers!

It contains the half chewed stone of a greenfruit. It has Tengu's saliva on it, maybe that's worth something, right?... Right?