Bayle the Ice Demon Lord

Ice Walk. The demon can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

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

Frozen Body: The Ice Demon takes double damage from fire-based attacks but also heals for 2 times the damage from any cold-based attacks they cannot damage itself, other Frost Fiends, and other ice demons.

You can Polymorph into this demon once a week. 

Multiattack. The demon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) cold damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Frightful Presence. Each creature of the demon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 14 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Cold Breath (Recharge 5–6). The demon exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 54 (12d8) cold damage on a failed save, or half as much damage on a successful one.

Ice Shard. Ranged Weapon Attack:  The Ice Demon summons shards of ice and levitates them in the air and uses his mind to throw them at the target +8 to hit, range 30/60 ft., one target. Hit: 35 (10d4 +1 ) Cold damage.

The dragon 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. The dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.