The Fresh crab shell is an uncommon drop from crabs, and can be crafted into a crab helmet with a chisel with 15 crafting, but there is a chance that the player will fail and result in a broken crab shell instead.
Recipe for Disaster must be partially completed for players to be able to access the Mogre Camp, where the crabs are located, and kill them.
Dropping monsters
Monster Combat level Quantity Rarity Crab 21; 23 1 Uncommon