Avanc

Hold Breath. The avanc can hold its breath for 30 minutes.

Keen Smell. The avanc has advantage on Wisdom (Perception) checks that rely on smell.

Surprise Attack. The avanc has advantage on attack rolls against any creature it has surprised.

Water Camouflage. The avanc has advantage on Dexterity (Stealth) checks made while underwater.

Legendary Resistance (3/Day). If the avanc fails a saving throw, it can choose to succeed instead.

Multiattack. The avanc makes three attacks: two with its bite and one with its tail.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 5) piercing damage.

Tail. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 22 (3d8 + 5) bludgeoning damage.

Whirlpool. Once per day, as an action, the avanc can create a whirlpool in a body of water it is in. The whirlpool is a 100-foot diameter circle centered on the avanc. A creature in the whirlpool's area must make a DC 15 Strength saving throw or be pulled 20 feet toward the center of the whirlpool, take 21 (6d6) bludgeoning damage, and be knocked prone. On a successful save, the creature takes half damage and isn't pulled. The whirlpool lasts for 1 minute or until the avanc uses an action to end it.

Swallow. The avanc makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the avanc, and it takes 21 (6d6) acid damage at the start of each of the avanc's turns. If the avanc takes 30 damage or more on a single turn from a creature inside it, the avanc must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate the swallowed creature, which falls prone in a space within 10 feet of the avanc. If the avanc dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.