A Crab Story - Act 4 Side Quest
Start Location: City of Rhodes
Quest Location: Damatria
Objective: Defeat the Gargantuan Karkinos
This quest begins by speaking with Arion (pictured above) in the City of Rhodes, the first city you come across in Act 4. You'll need to travel through the Cave of Orthea (shown on my map) to find a small region named Damatria. Speak with Proteas here to update your quest then head south to the beach and defeat the Gargantuan Karkinos.
It'll be a rather straight forward fight - he has no special mechanics just a larger crab that has more HP and hits harder. With all of that out of the way return to Proteas and speak with him again to turn in the quest and claim your reward.