Corotrangul, Lord of The Frigid Basin

Dew of Yggdrasil. Corotrangul has advantage on saving throws against spells and other magical effects. If Corotrangul succeeds on a saving throw against a spell or magical effect, Corotrangul regains 40 hit points.

Frigid Aura. The area within 100 ft. of Corotrangul is excessively cold. Liquid water within this area immediately freezes solid, and solid surfaces on the ground are immediately coated in a layer of rime, becoming Difficult Terrain. Each creature that starts its turn in this aura or moves into it for the first time on its turn must succeed on a DC 16 Constitution saving throw or take 13 (3d8) cold damage and fall Prone if it is on rime-covered ground.

Legendary Resistance (3/Day). If Corotrangul fails a saving throw, it can choose to succeed instead.

Multiattack. Corotrangul makes three Flood Strike attacks. It can replace any of them with its Coiling Tail. If it is available, it then uses its Ice Breath.

Flood Strike. Ranged Weapon Attack: +14 to hit, range 120 ft., one target. Hit: 17 (3d8+4) bludgeoning damage plus 9 (2d8) cold damage.

Coiling Tail. Melee Weapon Attack: +13 to hit, reach 80 ft., one target. Hit: 17 (5d6+3) bludgeoning damage, and the target is grappled (escape DC 19). While grappled in this way, a target is restrained. At the start of each of its turns, a grappled creature takes 7 (2d6) bludgeoning damage. Corotrangul's tail is long enough to grapple up to three creatures in this way.

Ice Breath (Recharge 5-6). Corotrangul releases a frigid burst of energy in a 120 ft. cone. Each creature within the cone's area must make a DC 18 Constitution saving throw, taking 67 (15d8) cold damage, or half as much on a success.

Abyssal Prayer (4/Day). Corotrangul energizes itself with ambient power. It regains 50 hit points, and the first attack that Corotrangul makes that hits a creature before it's next turn deals twice as much damage.

Corotrangul 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. Corotrangul regains spent legendary actions at the start of its turn.

Flood Strike. Corotrangul makes a Flood Strike attack.

Motion of Currents. Corotrangul moves up to its speed without provoking opportunity attacks. If, during this movement, Corotrangul moves within 5 ft. of a Large or smaller creature, that creature must succeed on a DC 14 Strength saving throw or fall Prone.

Aqua Veil (Costs 2 Actions). Corotrangul envelops its body in magical water. Until the start of its next turn, Corotrangul's vulnerability to fire damage is suppressed, and it instead gains resistance to fire damage.