Legendary Resistance (3/day).

Construct. The Undine is immune to poison damage, and immune to the poisoned and exhaustion conditions.

Ancient Body. The Undine has resistance to bludgeoning, piercing, and slashing damage from nonmagical sources.

Aquatic Semblance. The Undine is immune to cold damage and vulnerable to fire damage and is immune to the restrained condition while it is in water. If a creature the Undine is targeting is submerged in water, the Undine has advantage on attack rolls made against that creature.

Multiattack. The Undine attacks with two Energy Blade attacks.

Energy Blade. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 17 (2d8 + 8) force damage. 

Water Implosion (Recharge 5-6). While submerged in water, the Undine sends out a pulse of energy, causing the water around it to push out and pull back in an instant. All creatures within 30 feet of the Undine must make a DC 19 Dexterity saving throw, taking 88 (16d10) cold damage, or half as much on a failed save. Creatures that fail the save are thrown 20 feet from the Undine and are left with 30 seconds (5 rounds) of breath left in their lung capacity.

Aquatic Activation. As a Bonus Action while touching water, the Undine can wrap itself in energy, granting it a swim speed of 60 feet. This effect persists until the Undine is no longer in water.

Water Shield. When an attack will hit the Undine while it is in or within 10 feet of water, it can use its Reaction to shield itself using the water, granting +5 to its AC.