Calcifer, The Burning Titan

Intense Flames. When Calcifer deals fire damage from a spell he casted, he deals maximum damage.

Innate Spellcasting. Calcifer's spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: flame strike, fireball, immolation, wall of fire (each one at 5th lvl).

1/day each: incendiary cloud, meteor swarm, wish.

Legendary Resistance (3/Day). If Calcifer fails a saving throw, it can choose to succeed instead.

Extreme Heat (3/Day). When Calcifer casts a spell or uses an ability that deals fire damage, he can choose the damage to ignore fire resistance and to treat fire inmunity as if it were fire resistance.

Magic Resistance. Calcifer has advantage on saving throws against spells and other magical effects.

Unleash the Inferno (Mythic Trait; Recharges after a Short or Long Rest).
If Calcifer is reduced to 0 hit points, he doesn't die or fall unconscious. Instead, he regains 500 hit points and ends any condition, spell or similar effect affecting him. He also forces any creature within 20 feet of him to take 35 (10d6) fire damage and make a DC 23 Strength saving throw or be pushed 30 ft away from him.

Siege Monster. Calcifer deals double damage to objects and structures.

Multiattack. Calcifer makes three greatsword attacks or two rock attacks.

Incinerating Greatsword of Consumption. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 31 (6d6 + 10) slashing damage plus 21 (6d6) fire damage.

Smoldering Rock. Ranged Weapon Attack: +17 to hit, range 300/900 ft., each target on a 10 ft radius circle. Hit: 37 (5d10 + 10) bludgeoning damage plus 27 (5d10) fire damage.

Fire Breath (Recharge 5–6). Calcifer exhales fire in an 300-foot line that is 20 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 105 (30d6) fire damage on a failed save, or half as much damage on a successful one.

Stomp. Melee Weapon Attack: +17 to hit, reach 15 ft., one Large or smaller creature. Hit: 26 (6d6 + 10) bludgeoning damage and the target is grappled (escape DC 15). Until this grapple ends, the target is prone. The grapple ends early if Calcifer moves or makes another stomp attack.

Burning Rebuke. After getting hit by a melee attack, Calcifer can use his reaction to deal 33 (6d10) fire damage to the attacker.

Absorb Energy. If Calcifer takes radiant, acid, fire or lightning damage, he can use his reaction to negate the damage and gain temporary hit points equal to the damage taken. If the damage is fire, he instead gains double the temporary hit points

Calcifer can take 3 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. Calcifer regains spent legendary actions at the start of its turn.

Attack. Calcifer makes a stomp attack.

Leap. Calcifer moves up to half its speed.

Invoke Fire (Costs 2 Actions). Calcifer casts a spell.

Fire Breath (Costs 3 Actions). Calcifer uses its Fire Breath if available

lf Calcifer's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Unleash the Inferno.

Carbonizing Strike. Calcifer makes two greatsword attacks to the same target, if both attacks hit, the target must make a DC 23 Constitution saving throw or become petrified.

Hurl Rock. Calcifer makes a Smoldering Rock attack.