Water Devil

Devil's Sight. Magical darkness doesn't impeded the devil's darkvision.

Magic Resistance. The devil has advantage on saving throws against spells and other magical effects.

Amphibious. The devil can breathe air and water.

Innate Spellcasting. The Water Devil's spellcasting ability is Charisma (spell save DC 14, +6 to hit with spell attacks). The Water Devil has following spells prepared, requiring no material components:
At will:  Shape Water, Create or Destroy Water
3/day each: Fog Cloud, Ice Knife, Invisibility
1/day each: Fear, Tidal Wave, Wall of Water

Multiattack. The Water Devil makes two attacks: One with its claws, and one with its bite. While submerged in liquid, it can additionally use its tail attack.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage plus 9 (2d8) cold damage.

Tail. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 15 (2d10 + 4) bludgeoning damage.

Water Breath (Recharge 5–6). The Water Devil releases a torrent of water in a 20-foot long line that is 5ft. wide. Each creature in that line must make a DC 14 Strength saving throw. On a failed save, the creature takes 14 (4d6) bludgeoning damage and, if the target is large or smaller, be pushed away 10ft. in a straight line away from the devil and be knocked prone. On a success, the target takes half damage but isn't pushed away or knocked prone.