This is a support quest for She Who Controls the Seas quest. You will get it automatically when you approach the Underwater Cavern while doing the She Who Controls the Seas quest.
One of the maps given by Xenia for Triton's Conch Shell leads here.
Rumored Conch Shell Location
Objectives of Rumored Conch Shell Location
1. Explore the Underwater Cave at Chalkidike
2. Quest Completed
Rewards: Epic XP, Legendary Drachmae