Allura, the Corrupted Siren Matriarch

Hide in Plain Sight. Allura has advantage on Dexterity (Stealth) checks when around trees and any large slim structure, as her skin morphs and changes color slightly to match her surroundings.

Freezing Aura. Allura's body consistently emits a freezing mist. Every creature within 10ft of her must make a DC 17 Con saving throw. On a failure, the creature becomes restrained as ice crystallizes on its skin. A creature repeats the saving throw at the end of its turn. On a success, they are no longer restrained. If a creature fails this save three times in a row, they become petrified in ice. The creature remains petrified in ice until it leaves the aura, at which point it is restrained as it begins to thaw. At the end of its next turn, the creature is no longer restrained.

Shapechanger. Allura can alter her body to match her surroundings, but the transformation cannot change her size.

Magic Resistance. Allura has advantage on all saving throws against spells and other magical effects.

Multiattack. Allura makes two attacks, using either her Slam or Water Spear for each attack.

Slam. Melee Weapon Attack: +9 to hit, reach 5 ft.(range 20/60 ft.), one target. Hit: 27(4d10 + 5) bludgeoning damage.

Water Spear. Ranged Attack: +7 to hit, range 60 ft., one target. Hit: 20(4d8 + 2) cold damage. 

Detonating Bubbles (Recharges on a 6). Allura sends a bubble filled with concussive power at a creature it can see within 90 ft of her. That creature must make a DC 17 Dex save or take 66(12d10) force damage on a failed save, or half as much damage on a successful one.

Allura can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Allura regains spent legendary actions at the start of her turn.

Water Stride. If within water at least 1 ft deep, Allura can teleport to another area of water at least 1 ft deep within 90 ft.

Water Spear. Allura makes one Water Spear attack.

Water Clone (Costs 3 actions). If Allura is within 90 ft of a water source, she can create a water clone of herself. The clone has a speed of 0, AC 10, 65 hit points, and the same resistances and immunities as Allura. At the end of her turn, the clone makes a Water Spear attack at a creature within range. Allura can only sustain one clone at a time.