At the character screen, click Change Quest.
Under ACT III Chapter 5: Machines of War, select Destroy Ballistae. Click Select Game.
Click Start Game. Your character spawns at Bastion’s Keep Stronghold. Go to the waypoint; and under Field’s of Slaughter, select The Bridge of Korsikk.
You don’t need to go past the bridge at all. Roam around the Field’s of Slaughter. Usually to the northwest or far east side of the Field’s of Slaughter are small mounds. One out of several random Field’s of Slaughter maps contains a rare cave named Caverns of Frost Level 1.
On top of the random map containing the Caverns of Frost, there’s a random chance you might find a rare (yellow) Lacuni named Chiltara.
Chiltara has a random chance to drop the Gibbering Gemstone.