Aqua hullcus

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

Critical Freeze. If cold damage successfully hits the elemental the damage die are doubled, When the elemental takes cold damage, it partially freezes; its speed is reduced by 50 feet until the end of its next turn.

Siege Monster. The elemental deals double damage to objects, structures, and sea fairing vessels.

Water Form. The elemental can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Innate Spellcasting. The elemental’s innate spellcasting ability is Intelligence (save DC 15, +5 to hit with spell attacks). It can innately cast a number of spells, requiring no material components.

At will: Ice Storm, Ray of Frost, Cone of Cold

2/day each: Freezing Sphere, Prismatic Spray

1/day each: Storm of Vengeance

Multi-attack. The elemental makes two slam attacks.

Slam. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 30 (4d8 + 6) bludgeoning damage.

Whelm (Recharge 4–6). Each creature in the elemental's space must make a DC 16 Strength saving throw. On a failure, a target takes 30 (4d8 + 6) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 16). Until this grapple ends, the target is restrained and unable to breathe unless it can breathe water. If the saving throw is successful, the target is pushed out of the elemental's space.

The elemental can grapple one Large creature or up to two Medium or smaller creatures at one time. At the start of each of the elemental's turns, each target grappled by it takes 30 (4d8 + 6) bludgeoning damage. A creature within 5 feet of the elemental can pull a creature or object out of it by taking an action to make a DC 16 Strength check and succeeding.

The elemental can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The elemental regains spent legendary actions at the start of its turn.

Attack. The elemental makes one slam attack.

Pursuit. The elemental moves up to half its speed.

Tsunami (Costs 2 Actions). The elemental conjures up a tsunami.