Water A'aorca

Water aura. At the end of all of the elemental's turn each creature within a 15-foot emanation of the elemental take 15 (3d10) cold damage. 

Invisible in water. The A'aorca is invisible in water.

Water form. The A'aorca can go in another creature's space,

Legendary Resistance (3/day) or (4/day if in lair). If the A'aorca fails a saving throw it can choose to succeed instead.

Multiattack. The elemental makes 3 attacks of any kind.

Freezing hand. Melee Weapon Attack: +10 to hit, reach 10 ft. Hit: 26 (6d6 + 8) cold damage.

Extendable water. Ranged Weapon Attack: +14 to hit, range 10/50ft. Hit: 28 (9d4 + 10) cold damage.

Icy wall. The A'aorca makes a 50-feet wall of ice starting from where it is. The ice is 5-feet thick and has 50 hp.

Open hole. Trigger:  Hit by a ranged attack. Response: The A'aorca makes DC 17 saving throw. On a fail it takes the damage. On a success it opens a hole in its body letting the weapon through.

The A'aorca has 3 legendary actions (4 in lair) and it gains back any expended charges at the start of its turn. It can use a legendary action directly after another creature

Ray of frost. (Costs 2 Actions). Casts ray of frost at 17th level.

Commanding presence. The A'aorca casts command (level 2 version). It cant make this legendary action until its next go.

Puddle. When reduced to 0 hp the A'aorca will turn into a puddle which stays there until the next A'aorca is created.