Return to Cobblestone Tor during Act III to find a man has climbed to the top of the mountain. Speak to him to learn that he’s looking for a specific location. Sail to Insula Centralis (by way of the northern Vortex of Light) and mount a Malicious Dragooner to access the skies. Fly to the highest peak to the find a marker stone and then return to the man to complete the quest.