Stone is a level 27 boss and a General of the Black Ring, found at Stone's Flying Fortress in Orobas Fjords. After dealing with Xanlosch and Isabelle, if you use the nearby teleporter to the north, you'll enter the fortress' headquarters and confront Stone. After a skirmish he'll run and enter another teleporter to the south, which leads to the fortress arena. You'll have to contend with several creatures, including a troll, demon, goblin beholder, ghosts, and imps, before you can kill him.
Research notes written by Stone can be found in the fortress's Greenery, discussing his use of the magical fountain.