Argonix, The Hellfire Terror (Demolith)

Demolith Subtype. Argonix has the "(demolith)" tag, which has no rules of its own, but is a tag that affects him in the same way that it would affect a demon or devil.

Semi-Immortal Nature. Argonix can't die of old age unless affected by magic of a spell of 6th level or higher and doesn't require food, drink, air, or sleep.

Legendary Resistance (2/Day). If Argonix fails a saving throw, he can choose to succeed instead.

Lesser Magic Resistance. Argonix has advantage on saving throws against spells of 7th level or lower and similar magical effects.

Magic Weapons. Argonix's weapon attacks are magical.

Heated Fire. Fire damage dealt by Argonix bypasses fire resistance. Additionally, whenever he deals fire damage to a target that is immune to fire damage, the target does not take no damage and the amount of fire damage dealt by Argonix is reduced to 2.

Area Pyroshaper. After Argonix casts a spell or uses an ability that deals fire damage and covers an area (such as a fireball or fire storm spell), the spell or ability leaves a scorched area that remains for 1 minute. Whenever a creature starts its turn within the scorched area, it takes 5 (2d4) fire damage. Argonix's Fire Breath is not affected by this trait unless Argonix specifically uses it on the ground.

Fire Aura. Any non-Fiend creature that starts its turn within 15 feet of Argonix or is grappling him must succeed on a DC 17 Constitution saving throw or take 4 (1d8) fire damage. Additionally, the fire around Argonix creates bright light in a 5-foot radius and dim light for an additional 10 feet.

Multiattack. Argonix makes three melee attacks. He can replace one of the attacks with either a Leaping Slam or Ripping Swipe.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) slashing damage + 10 (3d6) fire damage.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 12 (1d12 + 6) piercing damage + 7 (2d6) fire damage.

Tail. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 11 (1d10 + 6) bludgeoning damage + 4 (1d8) fire damage.

Leaping Slam. Argonix leaps forward, landing with both hands clasped into a monstrous fist onto the ground at a point within 40 feet of his original location. Each target within 15 feet of Argonix when he lands must make a DC 16 Dexterity saving throw, taking 15 (2d8 + 6) bludgeoning damage + 11 (2d10) fire damage and be knocked prone on a failed save, or half as much damage and no other effects on a success.

Ripping Swipe. Argonix leans forward while swiping his two clawed hands in a large arc, forcing each target within 15 feet of him to make a DC 17 Dexterity saving throw, taking 15 (2d8 + 6) slashing damage + 7 (2d6) fire damage on a failed save, or quarter as much damage on a success.

Flame Burst. Argonix channels a fireball into his hand before slamming it down, causing an explosion in a 30-foot radius. Each target in that area must make a DC 17 Dexterity saving throw, taking 14 (4d6) fire damage + 4 (1d8) necrotic damage on a failed save, or half as much damage on a success.

Summon Demoliths (1/Short or Long Rest). Argonix summons 1d8+1 demoliths of CR 8 or lower at points he can see within 120 feet of him that remain for 3 minutes or until slain. The summoned demoliths cannot summon any other creatures, and take their turns immediately after Argonix.

Flaming Breath (Recharge 4-6). Argonix exhales fire in either a 40-foot cone or 70-foot line that is 10 feet wide. Each creature in that area must make a DC 19 Dexterity saving throw, taking 49 (14d6) fire damage on a failed save, or half as much damage on a successful one.

Innate Spellcasting. Argonix casts one of the following spells, using Charisma as his spellcasting ability (spell save DC 19, +11 to hit with spell attacks) without requiring material components. Argonix has the following spells prepared:

At will: fireball (8d6), continual flame, earth tremor, cause fear, Aganazzer's scorcher, jump

2/day each: fly (self only), arcane gate, wall of fire

1/day: fireball (11d6, 45-foot radius, range 90 feet)

3/day: fire storm

Argonix can take 2 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. Argonix regains spent legendary actions at the start of his turn.

Strike. Argonix makes a melee attack.

Charge. Argonix moves up to half his speed and has advantage on the next attack roll he makes.

Flame Plume (Costs 2 Actions). Argonix uses his Flame Burst.