Infernal Warrior

Based heavily on The Ironclad from Slay the Spire.

Infernal Warriors are fighters who have been touched by some hellish magic, which lIves in their fighting spirit, and comes alive when the fighter engages in combat.

Envelop yourself with flame and use your white hot fury to incinerate foes.

Ready Arms

Fiendish power envelops you and your weapon when you ready yourself to fight.

Limit Break: When you roll Initiative, Limit Break activates, upgrading your weapon’s damage die by 1, to a cap of 1d12. (Ex: 1d6 becomes 1d8, 1d8 becomes 1d10, but 1d12 stays the same. 2d6 becomes 1d12 if you wish.) You also gain a +2 bonus to either your Strength or Dexterity score, and it can go over the cap of 20. These effects end once the combat ends. 

Fiery Aspect: Additionally, you can use a Bonus Action during your turn to toggle your weapon’s damage type to fire. You can use a Bonus Action to toggle it off as well. While Fiery Aspect is toggled on, you can add 1d4 fire damage to all damage rolls.

Boiling Blood

At level 7, your infernal blood cleanses your body and empowers you.

Burn Corruptions: You become immune to disease, your sweltering body uninhabitable.

Play With Fire: You become resistant to fire damage. Any time you take fire damage, you heal an amount equal to your Constitution modifier, and your next attack will deal bonus fire damage equal to your Constitution modifier.

Fiendish Temper: During combat, while any crowd control effect is active on you (that has been inflicted upon you by an enemy), you may add +5 to all saving throws until you are free of their effects.

Immolation: Once per combat, you may cast Fireball as an action.

Fiend Fire

At level 10, your flames grow hotter, and take on a different color of your choosing, signifying that your fire now burns with new strength.

Fiend Fire: Whenever you deal fire damage, it ignores resistances and treats immunities as resistances. 

When your weapon has Fiery Aspect toggled on, it deals an extra 1d6 of fire damage rather than 1d4.

Engulf: When you activate your Fiery Aspect, you can now choose to also engulf yourself in flames. You take 1d8 fire damage at the start of your turn every round it is active, but while it is active, you deal (1d8 + Str or Dex modifier) fire damage to creatures within 5ft of you at the start of your turn. Enemies that melee attack you while this is active take 2d4 fire damage every time they hit you. You can choose to quell your fire at the end of your turn, ending Engulf. You can remain in your Fiery Aspect or return to your normal state.

Combustion

Combustion: While you are engulfed in flame, when you hit an enemy, you can choose to Combust, taking 2d8 fire damage to deal 2d12 fire damage to the enemy and any enemies within 10ft of it.

You may only use Combustion once per turn.

Cauterize: Whenever you take fire damage while you are at or below half of your maximum HP, you heal 5 HP.

Greater Immolation: Your Fireball is now 4th level.

Devil Form

At level 18, your flesh twists and molds you into a creature from the Hells, granting you infernal power.

Devil Form: When combat starts, you take on a fiendish nature, gaining powers.

You sprout wings and are granted a fly speed equal to your walk speed. 

Your weapon now deals an additional 1d8 of fire damage whether or not Fiery Aspect is active. 

When making damage rolls against enemies, rolls of 3 or lower become 4.

All self-inflicted fire damage is halved before calculating resistance.

You may cast a 5th level Fireball spell as an action once per combat.