Sando Aqua Monster

Armor Class 23 · Hit Points 546 (28d20 + 252) · Speed 20 ft., swim 50 ft.

Amphibious (Trait). The monster can breathe air and water.

Freedom of Movement (Trait). The monster ignores difficult terrain, and effects can't reduce its speed. It can spend 5 feet of movement to escape from being grappled.

Legendary Resistance (Trait). If the monster fails a saving throw, it can choose to succeed instead

Gargantan Strength (Trait). The monster's weapon attacks are considered enhanced.

Siege Monster (Trait). The monster does double damage on objects and structures.

Multiattack (Action). The monster uses its Frightful Presence. It then makes three attacks: one with its bite and two with its claws. It can use its Swallow instead of its bite.

Bite (Action). Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) kinetic damage. If the target is a creature it is grappled (escape DC 20). Until the grapple ends, the target is restrained, and the monster can't bite another target.

Claw (Action). Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 17 (2d6 + 10) kinetic damage.

Tail (Action). Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 19 (2d8 + 10) kinetic damage.

Frightful Presence (Action). Each creature of the monster's choice within 120 feet of the monster and aware of it must succeed on a DC 20 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each turn, ending the effect on itself on a success. If a creature's saving throw is successful or the effects ends for it, the creature is immune to the monster's Frightful Presence for 24 hours.

Water Torrent (Action). the monster exhales a torrent of water in a 120 foot cone. Each creature in the area must make a DC 24 Dexterity saving throw, taking 42 (12d6) kinetic damage and 42 (12d6) cold damage on a failed save, or half as much on a successful one.

Swallow (Action). If the monster takes 50 damage or more on a single turn from a creature inside it, the monster must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the monster. If the monster dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone. The monster makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the monster, and it takes 42 (12d6) acid damage at the start of each of the monster's turns.

Detect (Legendary). The monster makes a Wisdom (Perception) check.

Tail Attack (Legendary). The monster makes a tail attack.

Whirlpool (Legendary). A creature must make a DC 21 Dexterity saving throw the first time on a turn that it enters the whirlpool or that the whirlpool enters its space, including when the whirlpool first appears. A creature takes 6d8 kinetic damage on a failed save, or half as much damage on a successful one. In addition, a Large or smaller creature that fails the save must succeed on a DC 21 Strength saving throw or become restrained in the whirlpool until the power ends. When a creature starts its turn restrained by the whirlpool, the creature is pulled 5 feet deeper inside it, unless the creature is at the bottom. A restrained creature moves with the whirlpool and is submerged when the effect ends, unless the creature has some means to stay afloat. A restrained creature can use an action to make a DC 21 Strength or Dexterity check. If successful, the creature is no longer restrained by the whirlpool and is hurled 3d6x10 feet away from it in a random direction. The whirlpool ends at the beginning of the monster's turn unless the monster used the Whirlpool legendary action to maintain the whirlpool. The monster creates a whirlpool in the water at a point within 30 feet that it can see. The whirlwind is a 10-foot-radius, 30-foot-deep cylinder centered on that point. On subsequent turns, the monster can use the Whirlpool legendary action to maintain and move the whirlpool up to 30 feet in any direction along the surface of the water. The whirlpool sucks up any medium or smaller objects that aren't secured to anything and that aren't worn or carried by anyone.