Baalzebul, The Former Archon

Devil's Sight. Magical darkness doesn't impede Baalzebul's darkvision.

Decipher Weakness. Baalzebul scores a critical hit on a roll of 18 or higher.

Innate Spellcasting. Baalzebul's innate spellcasting ability is Charisma (spell save DC 24, +16 to hit with spell attacks). He can innately cast the following spells, requiring no material components:
Cantrips (at will): animate dead, detect evil and good, detect magic, fireball, hallow spell, hold monster, suggestion, teleport, true seeing, icy prison
3/Day each: dispel evil and good, symbol (pain or insanity), freezing sphere, empowered cone of cold
1/Day each: wish

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

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

Magic Weapons. Baalzebul's sword attacks are magical.

Regeneration. Baalzebul regains 20 hit points at the start of his turn. If he takes radiant damage, this doesn't function at the start of his next turn. Baalzebul dies only if he starts his turn with 0 hit points and is unable to regenerate.

Hellfrost. A type of cold magic that reaches Absolute Zero and is immune to all forms of fire, with the exception of Hellfire. All fiends take half damage before immunity to cold, to everyone else, cold Resistance is ignored and cold immunity turns into a resistance.

Multiattack. Baalzebul uses his Overwhelming Presence and then makes three attacks with the Sword of Baalzebul.

Sword of Baalzebul. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 23 (2d10 + 12) slashing damage, and 11 (3d6) necrotic damage. A creature that is hit by this sword cannot regain hit points for 1 minute. The target must succeed a DC 24 Constitution saving throw at the end of each of its turns, ending this effect.

Overwhelming Presence. Each creature of Baalzebul's choice that is within 240 feet of him and aware of him must succeed on a DC 24 Wisdom saving throw or be knocked prone, falling on their knees. Being charmed (if you chose awe) or frightened (if you chose fear). A creature that succeeds on this saving throw is immune to your aura for 24 hours.

Mass Suffocation (1/Day). Baalzebul extracts the air from all the creatures' lungs, causing swift suffocation. The targets must make a DC 24 Constitution saving throw; if a target succeeds, they are merely staggered until for 1 round as they gasp for breath. If a target fails, they immediately begin to suffocate. On the target's next turn, they fall unconscious and are reduced to 0 hit points. One round later, the target drops to death saving throws as they are dying. One round after that, the target dies. Each round, a target can delay that round's effects from occurring by making a successful Constitution saving throw, but the spell effect continues, and each time a target fails their Constitution saving throw, they move one step further along the track to suffocation. This spell only affects living creatures that must breathe.

It is impossible to defeat the effects of this spell by simply holding one's breath—if a target fails the initial saving throw, the air in their lungs is extracted. This prevents the target from speaking at all, reducing their ability to communicate and, preventing them from using any abilities requiring their voice. No Bardic music, no shouts of inspiration, no spells with verbal components.

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

Teleport. Baalzebul uses his Teleport action.

Suffocating Swarm (Costs 2 Actions). Baalzebul collapse into four swarm of flies in any arrangement he chooses and recorporate as needed. +16 to hit, reach 10 ft., one target. Hit: 21 (4d8 + 10) necrotic damage. If target(s) are hit, they become suffocated until Baalzebul's turn.