Water Aarakocra

Amphibious: The Water Aarakocra can breath in air and water.

Water form: The Aarakocra 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 Aarakocra takes cold damage , it partially freezes; its speed is  reduced by 20 feet until the end of it's next turn.

Spellcasting. The Water Aarakocra is a 20-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 13, +5 to hit with spell attacks). The Aarakocra has following spells prepared:
Cantrips (at will):  Dancing lights, Mage hand, Shape water
1st level (10 slots): Chromatic orb, Comprehend Language, Create and destroy water

Multiattack: The elemental makes two slam attacks.

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

Action Ranged Attack. RangedWeapon Attack: +# to hit, range #/# ft., # target. Hit: # (#d# + #) [damage type] damage.

Whelm (Recharge 4-6): Each creature in the Aarakocra'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 it also grappled (escape DC 14). Until this grapple ends, the target is restrained and unable to breath unless it can breath in water. If the saving throw is successful, the target is pushed out of the Aarakocra's space. The Aarakocra can grapple one Large creature or up to two Medium or smaller creatures at one one time with this ability. At the start of each of the Aarakocra's turn, each grappled target takes 13 (2d8 + 4) bludgeoning damage. As an action, a creature out of it by succeeding a DC 14 Strength check.