Literally a giant Eel. It measures over 100 ft long and is found in the rivers and lakes of the Underdark.
It attacks by whirling around in a single direction, causing a whirlpool to be created. This is used to drag it’s prey under the surface of the water. When attacking boats, it also jumps across the deck, attempting to knock creatures prone and to cover them in slime.
It was noted that loud bang of a gong disturb the creature, which prefers to escape the noise than to continue the fight.