Wave Master

Made from Water. Wave master can hide by blending into the water. It can only move on water terrain.

Cold Vulnerability. When taking cold damage Wave Master must make a Con saving throw (DC 10) or be petrified for 1 turn. When in this state instead of gaining resistance to all damage the damage received is doubled.

Multiattack. Wave Master attack three different targets with water bullet.

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage. 

Water Bullet. Ranged Weapon Attack: +6 to hit, range 60/120 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Hide. Wave Master can use its bonus action to hide under water.

Waves. If a creature attack Wave Master with melee attack, he summons 5 feet waves to sweep away his enemies, creature within 30 feet of it must succeed Dex saving throw (DC10) or will be pushed 5 feet and take 1d4 of bludgeoning damage, leaves a trail of water that he can step on that lasts for 1 turn.

Wave master can manipulate water around him to gain advantage at battle. His legendary action triggered after player's turn. He has legendary action equal to its proficiency, recover 1 action after its turn. Can't be activated on his turn.

Big Waves (Costs 2 Actions). Wave Master summons 10 feet waves to sweep away its enemies, creature within 60 feet of Wave Master must succeed Dex saving throw (DC14) or will be pushed 10 feet and take 1d6 of bludgeoning damage, half damage and not pushed on success, leaves a trail of water that he can step on that lasts for 2 turn.