Athel'gaxath, Demonlord

Legendary Resistance (10/Day): If the demonlord fails a saving throw, it can choose to succeed instead.

Siege Monster. The demonlord does five times as much damage to objects and structures.

Soul Absorption. The demonlord absorbs the soul of all creatures that it kills.

Destroyer of Worlds. If the demonlord absorbs ten million souls, it can destroy those souls to begin an evil corruption to spread from the core of one world that will spread and destroy the world in 30 minutes. 

Magical Attacks. The demonlord's attacks are magical.

 Frightful Presence: Each creature of the demonlord's choice that is within 200 feet of the demonlord and aware of it must succeed on a DC 25 Wisdom saving throw or become Frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 10 minutes.

Summon Minions. The demon lord summons 2d6 Barbed Devils.

Bite. Melee Weapon Attack: +19 to hit, reach 15 ft., up to 2 targets within five feet of each other. Hit: 150(20d10 + 50) piercing damage plus 150 (20d10 + 50) poison, acid, necrotic, or fire damage.

Claw. Melee Weapon Attack: +19 to hit, reach 20 ft., One Target. Hit: 200 (20d10 + 100) slashing damage plus 200 (20d10 + 100) poison, acid, necrotic, or fire damage.

Death Blast. The demonlord exhales a 20 ft. by 20 ft. ball of demonic energy up to 500 ft. Each creature within 50 ft. of where it hits must make a DC 24 Dexterity saving throw. A creature is knocked prone and takes 100 (5d20 + 50) poison, acid, necrotic, and fire damage on a failed save. A creature takes half as much damage on a successful save and is not knocked prone. The blast also explodes everything in a 100 ft. radius.

Can take 6 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. Spent legendary actions are regained at the start of each turn.

Regeneration. The demonlord regains 25 hit points.

Action. The demonlord uses any action.