After boarding the boat from the Underdark beach, a boat captained by Corsair Greymon will sail up alongside the party's boat. Greymon will come aboard, and the player character will have to pass a check or attack Greymon and his crew. Passing a DC 15 Athleticscheck to push him into the water will kill Greymon, sinking his inventory to the bottom of the sea, while initiating combat with the surviving Duergars; this will also garner approval from Lae'zel and Astarion.
If Greymon sails into the Grymforge with the party, he can later be found and traded with in the area where deep gnome slaves are digging out Nere.