Bael, Demon Lord of the Killing Frost

Infernal Heart. If this creature takes cold damage from an attack, it heals that much instead. if this creature is at full HP, it gains temporary hitpoints instead.

Demonic Regeneration. Bael regains 25HP at the start of each unless he took radiant damage since his previous turn. 

Aura of the Killing Frost. Baels presence calms the storm. a 60ft area surrounding Bael is cleared of the snowstorm. Any creature outside of this area takes 14 (4d6) cold damage, their movement is halved, and cannot regain hitpoints while not in the 60ft area. Whenever a creature takes cold damage from Bael, it gains one Frost Mark. At 3 Frost Marks, the creature becomes Frozen, and gains the following; the creature is restrained, becomes vulnerable to cold damage, and can repeat a DC 16 Constitution save at the end of each turn.

Twin Frost Maidens. Bael has two floating frost maidens that are attached to him. each of them has the following: AC15, 80HP and is resistant to all damage but force and radiant. they cannot be permanently destroyed, and when reduced to 0 hitpoints, they disappear. they reform after 2 rounds of combat. while both exist: Bael gains resistance to all damage, and gains advantage of all saving throws. 

Multiattack. Bael makes 3 attacks, 1 with his Claws, 1 with his Bite and 1 with his  Tongue Lash attacks.

Bite. Melee Attack, +12 to hit, reach 15ft, one target. Hit: 34 (4d10+10) Piercing damage + 18 (4d8) Cold damage. a creature that takes damage from this attack gains 1 Frost Mark.

Claws. Melee Attack, +12 to hit, reach 10ft, one target. Hit: 26 (3d10+10) slashing damage + 13 (3d8) cold damage. 

Tongue Lash. +14 to hit, reach 60ft, one target. Hit: 27 (4d8+9) bludgeoning damage. A creature hit by this attack must make a DC 20 Strength Saving Throw or be pulled 30ft towards Bael.

Glacial Devour (5 Charges). Bael exhales a storm of black frost in a 90ft cone. Any creature caught in the cone must make a DC 15 Constitution saving throw. on a failure it deals 43 (12d6) cold + 43 (12d6) Necrotic Damage, on a success they take half. creatures hit by this attack also gain 2 Frost Marks.

Consume the Frozen. Bael targets one Frozen creature within 60ft. the creature must make a DC 15 Constitution saving throw, on a failure the creature takes 55 Necrotic damage, and Bael heals the same amount. If a creature is reduced to 0 Hitpoints this way: The corpse of the creature transforms into an Ice wraith under Bael's control.

Bael has 4 Legendary Actions he can use a day, and they reset at dawn.

Legendary Resistance (4/day). If Bael fails a saving throw, he succeeds instead.

Frosty Step. Bael is able to move up to half his speed without provoking opportunity attacks. 

Tail Slam. Melee attack, +10 to hit, reach 15ft, one target. Hit: 26 (3d8+2) Bludgeoning damage. 

Ice Spike (2 Actions). A ice spike erupts from the ground beneath a creature that is within 120ft. the creature must make a DC 16 Dexterity saving throw, on a fail the creature takes 24 (5d6) piercing damage + 24 (5d6) cold damage. If a creature takes damage from this attack their speed is reduced to 0 until their next turn.

Soul Blizzard (3 Actions). Every creature within 80ft of Bael must make a DC15 Dexterity saving throw. on a fail it deals 18 cold damage + 18 slashing damage, and also becomes frightened of Bael until their next turn. 

The Heart of Winter. When Bael's hitpoints are reduced to 0, he does not die. Instead the entire battlefield is coated in frost, the moon turns blood red, and Bael erupts with demonic ice. Bael regains 350HP, any conditions on him are removed, and his Frost Maidens reform at full hitpoints. 

Beast of the Frozen End. Bael's movement is increased to 60ft, the Aura he is emitting decreases by 20ft, and he gains the following;

Icequake (5 charges). The ground within 120ft of Bael erupts. Every creature must make a DC 17 Dexterity saving throw, on a failure a creature takes 32 (12d6) cold + 32 (12d6) bludegeoning damage. the creature is also knocked prone. 

Death Blizzard. Whenever a Character falls unconsious; everyone within 30ft of the character takes 18 cold damage, and Bael heals 25 hitpoints.

Legendary Action: Absolute Zero (4 Actions). Every creature with Frost Marks immediately makes a DC 18 Constitution saving throw, on a failure a creature becomes Frozen and takes 36 (4d6+4) cold damage.