This is a Side Quest in Hades (Act IV). A giant crab causes trouble to the fishermen in Damatria, hindering them from fishing.
Talk to Arion next to the portal in the City of Rhodes to receive this quest. To complete it, simply follow the path to the east. Then go north to the Cave of Orthea.
When you exit the cave to the west again, you'll find yourself in Damatria. Speak to Proteas directly south of the exit. He will tell you about the Gargantuan Karkinos on the beach to the southwest. Kill it and turn in the quest with Proteas.