Berial, Demon Lord of the Hellforged Flame

Infernal Heart. If this creature takes fire damage from an attack, it heals that much instead. if this creature is at full HP, it gains temporary hitpoints instead.

Aura of the Hellforged Flame. Berials presence commands the flame to his will. a 60ft area surrounding Berial is cleared of the inferno. Any creature outside of this area takes 14 (4d6) fire damage, their movement is halved, and cannot regain hitpoints while not in the 60ft area. Whenever a creature takes fire damage from berial, it gains one Burning Brand. At 5 Brands, a creature takes 21 (6d6) fire damage, cannot benefit from invisibility, and has disadvantage on Constitution saves. A creature may remove all Brands by spending an action to put out the flames or by receiving magical cold damage.

Lord of the Infernal Forge. Fire created by Berial ignores Fire Resistance. Creatures immune to Fire instead treat it as Resistance. Only complete immunity from magical artifacts or divine effects prevents his flames entirely.

Magmaquake. When Berial uses his charge attack, he leaves a trail of infernal flames in his step. Any creature caught in the flames takes 22 (4d6) Fire Damage. 

Molten Blood. When a creature deals damage to Berial with a melee attack, that creature takes 10 (2d6) fire damage.

Multiattack. Berial makes 4 attack, 2 with his Infernal Claws and 2 with his Ignis, Blade of the Inferno.

Infernal Claws. Melee Attack, +12 to hit, reach 10ft, one target. Hit: 18 (2d8) slashing damage + 10 (3d4) fire damage. A creature hit by this attack must make a DC 14 Strength saving throw or be pushed back 10ft. 

Ignis, Blade of the Inferno. Melee Attack, +12 to hit, reach 15ft, one target. Hit: 30 (3d8+10) slashing damage + 18 (3d6) fire damage. A creature that takes damage from this attack gains 1 Burning Brand.

Infernal Charge. Berial lowers his head, and violently gallops his entire movement speed in a single line. Any creature hit by this attack must make a DC 14 Consitituion saving throw, on a fail they take 20 (2d8) piercing damage + 8 (1d6) fire damage.

Flaming Meteor (5 charges). Berial jumps into the air up to his movement speed and then crashes down into the ground, releasing a wave of fire, magma and rock. Any creature that is within 30ft of this attack must make a DC 16 Dexterity saving throw, on a fail they take 26 (3d8) fire damage + 18 (2d8) bludgeoning, on a success the damage is halved. This attack generates 1d4 immovable boulders on the battlefield.

Berial has 4 Legendary Actions he can use a day, and they reset at dawn.

Legendary Resistance (4/day). If Berial fails a saving throw, he succeeds instead.

Ignis Wildfire (2 actions). Ranged attack, +12 to hit, reach 30ft cone, multiple targets. Hit: 45 (4d10) fire damage. A creature hit by this attack gains 2 Burning Brands. 

Branded Detonation (2 actions). Berial unleashes a devastating roar, sending a shockwave through the battlefield and detonating all brands that a creature has. For each brand a creature has, it takes 1d8 fire damage ignoring resistance and treating immunity as resistance. 

Volcanic Rift (3 actions). Berial raises onto his hind legs and then crashes down into the ground below him with his front legs, creating 3 massive tears that run through the battlefield that are full of magma. Any creature that stands in the magma takes 16 (2d8) fire damage. 

Wrath of the Inferno. When Berial’s hitpoints are reduced to 0, he does not die. Instead the entire battlefield erupts into an inferno, chunks of molten rock rain down from the skies, and Berial is consumed in hellfire. Berial regains 450HP, any conditions on him are removed, and all Burning Brands on creatures turn into Hellfire Brands. 

Hellfire Dominance. Whenever Berial deals damage to a creature that has at least 1 Hellfire Brand on it, every creature that is within 30ft of the attacked creature takes 10 (2d6) fire damage. 

Critical Prowess. Whenever Berial makes an attack against a creature, it is classed as a Critical hit on a Nat 19 instead of a Nat 20.

Legendary Action (4 actions) Apocalyptic Surge. Berial unleashed a supernova of hellfire that coats the entire battlefield. Any creature caught in the blast must make a DC 20 Constitution saving throw, on a fail a creature takes 130 (12d10) fire damage. On a success they take half. A creature that is stood behind an immovable boulder does not need to make this save, and automatically takes half the damage. Creatures hit by this attack gain 1 Hellfire Brand.