Black Blizzard (Banshee)

Detect Life. Black Blizzard can magically sense the presence of creatures up to 5 miles away that aren’t undead or constructs. She knows the general direction they’re in but not their exact locations.

Incorporeal Movement. Black Blizzard move through other creatures and objects as if they were difficult terrain. She takes 5 (1d10) force damage if she ends her turn inside an object.

Heart of the Storm. Black Blizzard is surrounded by cold winds. All non-closed light sources within 60 feet of Black Blizzard are extinguished and all liquids within 20 feet are frozen if there are no creatures on them, as well as at 15 feet the wind becomes too strong and any creature that does not have a cold resistance is considered to be on difficult terrain.

Dry ice. Black Blizzard is as hard as ice, but after a while its protection can still be broken through. She takes damage only at the beginning of her turn. If the sum damage was less than 50 before the start of her turn, then Black Blizzard ignores it. But if the sum damage was more than 80, then she also ignores it and acquires resistance to all types of damage that were inflicted on her until the end of her current turn.
If the damage was inflicted during her turn, then it counts as if it was inflicted after her turn

Spellcasting. Black Blizzard is an 13th-level spellcaster. Hers spellcasting ability is Charisma (spell save DC 19, +11 to hit with spell attacks). Black Blizzard has the following Warlock prepared:  (you can choose any others spells, but I think it's more acurate to be Warlock spells)

Cantrips (at will): Thunderclap, Frostbite, Toll the dead

1st level (4 slots): Hex, Cause fear, Arms of Hadar

2nd level (3 slots): Darkness, Hold person, Ray of enfeeblement

3nd level (3 slots): Dispel magic, Thunder step

4nd level (3 slots): Shadow of Moil, Charm monster

5nd level (2 slots): Danse macabre, Mislead

6nd level (1 slots): Scatter 

7nd level (1 slots): Forcecage

Multiattack. Black Blizzard can use its Cruel Cold. It then makes three attacks with Corrupting Touch.

Corrupting Touch. Melee Spell Attack: +11 to hit, reach 5 ft., one target. Hit: 21 (2d10 + 6) necrotic damage and 9 (3d6) cold damage.

Cruel Cold. Any creature within 60 feet of Black Blizzard that can see her receive 3d6 cold damage for each level of exhaustion.  (for example: if the creature already has 2 levels of exhaustion, it receives 6d6 cold damage).

Wail (3/Day). Black Blizzard releases a mournful wail, provided that she isn’t in sunlight. This wail has no effect on constructs and undead. All other creatures within 60 feet of her that can hear her must make a DC 19 Constitution saving throw. On a failure, a The creature gets 1 level of exhaustion. On a success, a creature takes 24 (4d12) psychic damage.

Black Blizzard 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. Black Blizzard regains spent legendary actions at the start of hers turn.

Cantrip. Black Blizzard casts a cantrip.

Corrupting Touch (Costs 2 Actions). Black Blizzard use Corrupting Touch.

Cot of Cold (Costs 2 Actions). Black Blizzard selects any creature within 60 feet of itself and wraps it in a strong wind. The selected creature must succeed on a DC 19 Wisdom saving throw or its speed will drop to 0. With success, nothing happens.

Cruel Cold (Costs 2 Actions). Black Blizzard use Cruel Cold.