Aqua Madoor

 Innate Spellcasting. The Aqua Madoor is a 8th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 14, +7 to hit with spell attacks). It has following spells prepared:

1st level (2 slots): Fog Cloud, Armor of Agathys
2nd Level (3 slots): Mirror Image, Misty Step
3rd Level (3 slots): Tidal Wave, Wall of Water
4th Level (1 slot): Watery Sphere

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.

Freeze. If the elemental takes cold damage, it partially freezes; its speed is reduced by 20 feet until the end of its next turn.

Multiattack. The elemental makes two slam or ice beam attacks, or Tidal Surge or Whelm if available.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Ice Beam. Ranged Magic Attack. +7 to hit, range of 45 ft, 1 target. Hit: 10 cold damage, and targets movement speed is reduced by 10 feet until end of its next turn. 

Tidal Surge (Recharge 5-6). Enemies in a 30 foot cone in front of you must make a DEX save DC14, or take 32 [4d8] magical bludgeoning damage and be knocked prone. A successful save takes half damage and is not knocked prone. 

Whelm (Recharge 4–6). Each creature in the elemental's space must make a DC 15 Strength saving throw. On a failure, a target takes 13 (2d8 + 4) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 14). 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 13 (2d8 + 4) 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 14 Strength check and succeeding.