Bel'akor the Prime evil

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

Undying. If Bel'akor is slain, his soul refuses to accept its fate and lives on as a disembodied spirit that fashions a new body for itself after 500 years. Bel'akor soul can fashion a new body even if its old body was burned to ash or otherwise obliterated. When the new body is complete, Bel'akor regains all his hit points and becomes active again. Bel'akor new body appears anywhere within 100 miles of where Bel'akor was slain..

Death Throes. When the Bel'akor dies, it explodes, and each creature within 30 feet of it must make a DC 20 Dexterity saving throw, taking 70 (20d6) fire damage and (20d6) necrotic damage.on a failed save, or half as much damage on a successful one. The explosion ignites flammable objects in that area that aren’t being worn or carried, and it destroys the Bel'akor weapons.

Fire Aura. At the start of each of the Bel'akor turns, each creature within 15feet of it takes (8d6) necrotic damage,and (8d6) fire damage, and flammable objects in the aura that aren’t being worn or carried ignite. A creature that touches the Bel'akor or hits it with a melee attack while within 5 feet of it takes 10 (8d6) fire damage.

Magic Resistance. The Bel'akor has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Bel'akor weapon attacks are magical.

Petrifying Gaze. When a creature that can see the Bel'akor eyes starts its turn within 30 feet of the Bel'akor, Bel'akor can force it to make a DC 14 Constitution saving throw if the Bel'akor isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise, a creature that fails the save begins to turn to glass and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic.

Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the Bel'akor until the start of its next turn, when it can avert its eyes again. If the creature looks at the Bel'akor in the meantime, it must immediately make the save.

If Bel'akor sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, Bel'akor is, due to its curse, affected by its own gaze.

Its Petrifying Gaze feature turns petrified creatures to glass instead of stone.

Multiattack. Bel'akor makes three attacks: two with his greatsword and one with his finger of doom..

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 12 (3d6 + 9) slashing damage. If the target is a Huge or smaller creature, it is knocked prone.

Greatsword. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 23 (4d6 + 9) slashing damage plus 21 (6d6) fire damage. If the target is a flammable object that is not being held or worn, it catches fire.Additionally, any Medium or smaller Humanoid killed by this damage, as well as every corpse of such a creature within the blades radius 10 ft., becomes a skeleton (see the Monster Manual) under Bel'akor control. The skeleton acts on Bel'akor initiative but immediately after his turn. Absent any other command, the skeleton tries to kill any non-Undead creature it encounters.

Finger of Doom . Bel'akor points at one creature it can see within 300 feet of it. The target must succeed on a DC 21 Wisdom saving throw or take 26 (4d12) necrotic damage and become frightened until the end of the Bel'akor next turn. While frightened in this way, the creature is also paralyzed. If a target’s saving throw is successful, the target is immune to the Bel'akor Finger of Doom for the next 24 hours.

Cataclysmic Fire (4/Day). Bel'akor hurls a magical ball of fire that explodes at a point he can see within 120 feet of himself. Each creature in a 20-foot-radius sphere centered on that point must make a DC 19 Dexterity saving throw. A creature takes 35 (10d6) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.

Additionally, any Medium or smaller Humanoid killed by this damage, as well as every corpse of such a creature within the sphere, becomes a skeleton (see the Monster Manual) under Bel'akor control. The skeleton acts on Bel'akor initiative but immediately after his turn. Absent any other command, the skeleton tries to kill any non-Undead creature it encounters.

Cataclysmic Breath (4/Day). Bel'akor exhales a wave of ghostly purple flames in a 60-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 45 (10d8) necrotic damage on a failed save, or half as much damage on a successful one. A creature dies if the breath reduces it to 0 hit points. Additionally, any Medium or smaller Humanoid killed by the breath’s damage, as well as every corpse of such a creature within the cone, becomes a zombie (see the Monster Manual) under Bel'akor control. The zombie acts on Bel'akor initiative but immediately after the Bel'akor turn. Absent any other command, the zombie tries to kill any non-Undead creature it encounters.

Hideous Rend. Bel'akor uses all four of its hands to target one Large or smaller creature it can see within 10 feet of itself. The target must succeed on a DC 20 Dexterity saving throw or be grappled (escape DC 20). Until this grapple ends, the Bel'akor can’t make Fist attacks or Rock attacks, and the target takes 49 (8d10 + 5) bludgeoning damage at the start of each of its turns. A creature reduced to 0 hit points by this damage is ripped into four pieces.

 Lightning Flash . Bel'akor moves up to half his flying speed without provoking opportunity attacks. 

Bel'akor  can take up to three reactions per round but only one per turn.

Dread Counterspell. Bel'akor utters a dread word to interrupt a creature he can see that is casting a spell. If the spell is 4th level or lower, it fails and has no effect. If the spell is 5th level or higher, Bel'akor makes an Intelligence check (DC 10 + the spell’s level). On a success, the spell fails and has no effect. Whatever the spell’s level, the caster takes 10 (3d6) psychic damage if the spell fails.

Fell Rebuke. In response to being hit by an attack, Bel'akor utters a fell word, dealing 10 (3d6) necrotic damage to the attacker, and Bel'akor teleports, along with any equipment he is wearing or carrying, up to 30 feet to an unoccupied space he can see.

Call the Void (Costs 6 Actions). Each creature that is not a construct or undead within 30 feet of Bel'akor must make a DC 22 Constitution saving throw against Bel'akor attempt to unmake life, taking 42 (25d10) necrotic damage on a failed save, or half as much damage on a successful one. Bel'akor then regains a number of hit points equal to half the total damage taken by all affected creatures.