You will receive experience equivalent to 5000 times your level by handing this stone to Milos (here) or Trimegis (here). As of August 29 2012, you can only use one stone per year, and only when the Rise of Devovorga event is active (September 01 - 07). On worlds which were successful players may use this stone up until September 30. Click Here to Show/Hide Spoiler Information
Spoiler warning: Quest and/or game spoiling details follow.