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 to convince him to let you through to Grymforge. If the party is unable to pass the check, or they choose violence, Greymon will attack and Jerris will join the battle.
Your attack summons thorny vines, dealing Weapon Damage and possibly Ensnaring your target.
Ensnared creatures cannot move and take 1d6Piercing damage at the start of each turn. An ally can use their Help Action to try and tear away the vines.