Beelzebub, the Lord of Flies

 Beyond Powerful. Beelzebub, the Lord of Flies, is a Daemon Lord, and one of the four children of the Demon King. His CR is actually CR 32. Due to this terrifying power, Beelzebub stats go beyond 30, leading to them being the following.
Strength: 30 (+10) [Save: +13]
Dexterity: 24 (+7) [Save: +19]
Constitution: 33 (+11) [Save: +23]
Intelligence: 30 (+10) [Save: +22]
Wisdom: 28 (+9) [Save: +21]
Charisma: 30 (+10) [Save: +13]

Legendary Resistance (4/Day). If Beelzebub fails a saving throw, he can choose to succeed instead.

Blighted Weapons. Beelzebub's weapon attacks are treated as +3 magical weapons (included in the attack). When Beelzebub hits with any weapon, the weapon deals an extra 18 (4d8) Necrotic Damage (included in the attack).

Banishment.  When Beelzebub drops to 0 hit points, his body explodes into a cloud of biting insects. As long as Frigidum, the Demon King, is still alive, Beelzebub can reform even when killed in Cocytus.

Limited Magic Immunity. Beelzebub can't be affected or detected by spells of 4th level or lower, unless he wishes to be. He has advantage on saving throws against all other spells and magical effects.

Acid and Poison Absorption. Whenever Beelzebub is subjected to acid and poison damage, he takes no damage and instead regains a number of hit points equal to the acid and poison damage dealt.

Lord of Flies. Beelzebub can command all insects within 1 mile of him as if he had cast the spell Dominate Beast on them, except only needed to spend a bonus action to exert precise control. In addition, while an insect is within 120 feet of him it gains a +10 bonus to its attacks and they are considered magical.

Flyby. Beelzebub doesn't provoke opportunity attacks when he flies out of an enemy's reach.

Quickened Senses. Beelzebub has advantage on Initiative. 

Regeneration. Beelzebub regains 60 hit points at the start of each of his turns.

Flies. A special condition given when a creature is hit by Beelzebub and they fail a Constitution saving throw. While the target has the Flies condition, they can not gain any benefit from a Magical Aura, like that from a Paladin's. They are also treated as if being effected by the Poison Condition. This effect ignores a creatures Immunity to the Poison Condition.
Those with the Fly Debuff can be effected by Beelzebub's Lair Action: Death Flies.
This condition ends if a target uses any magical healing above 5th Level, or Greater Restoration.

Limited Magical Defenses. Beelzebub resists magical bludgeoning, piercing, and slashing damage, however this is ignored if it is a +3 Magical Weapon or above.

Critical Talent. Beelzebub's critical hit range for both weapon attacks and spell attacks is 18 - 20. In addition, when Beelzebub crits on Melee Weapon attacks, the damage dice is increased by 3.

Global Benefit. Beelzebub innately has a +3 to all Melee and Ranged Weapon Attacks and Damage, +3 to all Melee and Ranged Spell Attacks and Damage, and a +3 to all Saving Throws, Skills, and to his Spell DC. All of these are already included.

Staff of Apomyuis. This staff in the hands of Beelzebub is a +4 Epic Magic Item, that deals an extra 28 (8d6) Poison damage, included in the attack. In addition, the staff has 10 charges, and any of its properties that require a saving throw uses Beelzebub's. While holding it, he can use a bonus action to spend 1 or more charges to cast one of the following spells at 7th level:
At Will: Banishing Smite (2 Charges), Blinding Smite (1 Charge), Staggering Smite (1 Charge)
The Staff regains 1d6+4 charges daily.

Innate Spellcasting. Beelzebub's innate spellcasting ability is Intelligence (spell save DC 30, +22 to hit with spell attacks). Beelzebub can innately cast the following spells and upcast them at 7th level innately, requiring no material components:
At Will: Charm Person, Detect Magic, Dispel Evil and Good, Dispel Magic, Fear, Giant Insect, Hold Monster, Insect Plague, Invisibility, Poison Spray (6d12), Suggestion, Teleport, Wall of Fire
4/Day each: Dominate Monster
3/Day each: Blight (9th Level), Blindness/Deafness, Hellish Rebuke, Locate Object, Polymorph, Shapechange, Symbol
2/Day each: Time Stop
1/Day each: Abi-Dalzim's Horrid Wilting, Feeblemind, Power Word Kill, Unholy Aura, Wish

Multiattack. Beelzebub makes three attacks, any combination of his Staff, Horrifying Gaze and Withering Touch.

Staff of Apomyuis. Melee Weapon Attack: +26 to hit, reach 15 ft., 1 target. Hit: 66 (14d6 + 17) Bludgeoning damage, plus 18 (4d8) Necrotic damage, plus 28 (8d6) Poison damage.
On a hit, the target must make a DC 30 Constitution saving throw. On a fail, the target is effected with the Flies Condition.

Withering Touch. Melee Spell Attack: +19 to hit, reach 10 ft., 1 target. Hit: 63 (14d8) Necrotic damage.
On a hit, the target must make a DC 30 Constitution saving throw or suffer a level of exhaustion (this effect stacks). The  targets maximum hit point is reduced by an amount equal to the necrotic damage taken, and Beelzebub regains hit points equal to the amount. The hit point reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced to 0.

Horrifying Gaze. Ranged Spell Attack: +22 to hit, range 480 ft., 1 target. Hit: 42 (4d20) Psychic damage plus 18 (4d8) Necrotic damage.
On a hit, the target must make a DC 30 Wisdom saving throw. On a fail, the target is frightened and has disadvantage on Strength and Dexterity Saving Throws, and all attacks do half damage. The target may make an additional saving throw at the end of each of its turn, ending the effects on a success. Beelzebub has advantage on attacks against targets frightened by this attack.

Summon Devil (3/Day). Beelzebub magically summons 1d4+2 Horned Devils.

Staff Spell. Beelzebub can cast one of his Staff of Apomyuis' spells.

Beelzebub can take 4 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. Beelzebub regains spent legendary actions at the start of his turn.

Strike (Costs 1 Action). Beelzebub makes one Staff attack, Horrifying Gaze, or Withering Touch Attack.

Cast a Spell (Costs 1 Action). Beelzebub casts one of his Innate spells.

Teleport (Costs 2 Actions). Beelzebub magically teleports, along with any equipment he is wearing or carrying, up to 120 feet to an unoccupied space he can see.

Dance of the Flies (Costs 3 Actions). Beelzebub uses his Staff of Apomyius to unleash a wave of Flies. This is a 60 foot radius centered on Beelzebub. Any in it must make a DC 30 Charisma Saving Throw. Those with the Flies Debuff automatically fail.

Those that fail will feel their insides being eaten from the inside out, and they are stunned and paralyzed in agony (this ignores condition immunity). They also gain 100 Blight Temp HP. If they do not get rid of this Temp HP in in 2 rounds, they will take the Blight Temp HP as damage which pierces resistances and immunities.