Water Primordial

Regeneration: The Water Primordial regains 30hp at the start of each of its turns. 

Limited Magic Immunity: Unless allowed, the Water Primordial is immune to spells 6th level or lower. Has advantage on all other saving throws against spells and magical effects. 

Magical attacks: Water Primordial's attacks are magical

Waterform: The Primordial can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing. 

Tsunami (Recharge 5-6): As an action, the Water Primordial can make the effects of the Tsunami spell. DC22

Multiattack: The Water Primordial can make five slam attacks and a Whelm attack as an action. 

Slam: Melee weapon attack : +19 to hit, reach 10ft, Hit: 38 (4d12+10) Bludgeoning damage 

Whelm (Recharge 4-6): Each creature in the Primordial's space must make a DC 22 Strength saving throw. On a failure, a target takes 21(3d12) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 18). 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 Primordial's space.
The Primordial can grapple one Huge creature, two Large creatures, or up to four medium creatures. At the start of each of the Primordial's turns, each target grappled by it takes 21(3d12)bludgeoning damage. A creature within 5 feet of the Primordial can pull a creature or object out of it by taking an action to make a DC 22 Strength and succeeding.  

The Water Primordial can take 5 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 Primordial regains spent legendary actions at the start of its turn. 

Slam: Melee weapon attack : +19 to hit, reach 10ft, Hit: 38 (4d12+10) Bludgeoning damage 

Waverider: The Water Primordial can move its full movement.

Geyser: The Primordial can make a geyser 10ft in diameter and 40ft high. Any creature caught the radius makes a DC18 Dexterity save or be launched into the air.