Bladewarrior of the Flame

Your innate magic manifests as fire. You are your fire, and the fire is you.

As your blade cuts your enemy, your innate fire manifests and ignites the blade and burns your enemy.

Heart of Fire

Beginning when you choose this archetype at 3rd level, your weapon attacks deal additional fire damage worth to halve the damage already dealt.

This feature only works with weapons that deal slashing damage.

Fire in the Veins

Starting at 7th level, you gain resistance to fire damage. In addition, your attacks ignore resistance to fire damage.

Ignited Fury

At 10th level, when you are hit by a melee attack, you can use your reaction to deal fire damage to the attacker. The damage equals to your fighter level, and ignores resistance to fire damage.

Fiery Soul

Starting at 15th level, you gain immunity to fire damage. In addition, any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.

Explosive downfall

At 18th level, when you fall to 0 hit points, your fire will manifest in the form of  an explosion of flame.

Each creature in a 20-foot radius must make a Dexterity saving throw. A target takes 14d6 fire damage on a failed save, or half as much damage on a successful one. The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried.

You can use this feature once per long rest.