Baalzebul

Augmented critical. When Baalzebul scores a critical hit it deals triple the normal damage. 

Aura of stench. At the end of each of Baalzebul's turn each creature within 20 ft must make a dc 22 Constitution saving throw or be poisoned for 1 minute. 

Cloud of flies. Instead of the disgorge insects attack Baalzebul can choose to form a cloud of flies making so any ranged attack has a 20% chance of missing. 

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

Magic Weapon. Baalzebul's weapon attacks are magical. 

Innate Spellcasting. Baalzebul’s innate spellcasting ability is Intelligence (Dc 25, +17). He can innately cast the following spells, requiring no material components:

• At will: fireball, geas, dispel magic, scrying, teleport, hold monster, insect plague, major image, suggestion, true seeing
• 1/day each: abi-dalzim's horrid wilting

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.

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

Multi attack. Baalzebul makes either two slam attacks or a withering touch attack and a gaze of fear attack. 

Slam. Melee Weapon Attack: +17 to hit, reach 10 ft., 1 target. Hit: 27 (3d12 + 9) Bludgeoning damage. 

Withering touch. Melee Weapon Attack: +17 to hit, reach 10 ft., 1 target.  On a hit, the target deals only half damage with weapon attacks that use Strength until the spell ends.
At the end of each of the target’s turns, it can make a Dc 22 Constitution saving throw. On a success, this effect lasts for 1 minute. 

Fear gaze. One creature that can see Baalzebul of Baalzebul's choice must make a dc 22 Wisdom save or be frightened for 1 minute. 

Summon Devil (2/Day) Baalzebul summons 2 horned devils or 2 ice devils. A summoned devil appears in an unoccupied space within 60 feet of Baalzebul, acts as an ally of Baalzebul, and can’t summon other devils. It remains for 1 minute, Baalzebul dies, or until its summoner dismisses it as an action. 

Baalzebul 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. Baalzebul regains spent legendary actions at the start of his turn.

Weakening gaze (Costs 1 Actions). One creature of Baalzebul's choice within 60 ft of him that can see him must make a dc 23 Constitution saving throw or have a -3 penalty to all skill checks saves and attack rolls that use Dexterity and strength until the end of their next turn. 

Disgorge insects (Costs 2 Actions). Baalzebul breaths a 60ft cone of poisonous insects and each creature within the range must make a dc 23 Constitution saving throw or take 40 (10d8) poison damage. 

Stuck like slugs (Costs 3 Actions). Each creature within 120 ft of Baalzebul must make a dc 23 Constitution saving throw or have their speed turned to 5 ft and not able to dash and only be able to take an action or a bonus action and no reactions until the end of their next turn.