Fighter subclass: Crucible Knight

Level 3 — Forge-Bound   Your connection to fire grants:   Fire Resistance: You gain resistance to fire damage while not wearing heavy armor that prevents mobility.   Inferno Strike: Once per turn, when you hit with a weapon attack, the attack deals 1d4 fire damage. This damage scales to 1d6 at level 10, 1d8 at level 15.   Level 7 — Living Furnace   When you use Second Wind, creatures of your choice within 5 ft of you take fire damage equal to your Fighter level.   The effect has no save, but you may choose creatures to avoid friendly fire.   If your DM prefers saving throws: creatures can make a Dexterity save (DC = 8 + your Proficiency Bonus + your Strength or Dexterity modifier) to take half damage.   Level 10 — Molten Weapon   As a bonus action, you can ignite one weapon you are holding.   The weapon deals 1d6 fire damage on a hit for 1 minute, stacking with Inferno Strike.   You can use this feature once per short or long rest.   The fire damage scales 1d8 at level 15, 1d10 at level 18.   Level 15 — Crucible Armor   Your armor radiates the heat of the forge.   Creatures that hit you with melee attacks take fire damage equal to your proficiency bonus.   This is a reaction or automatic trigger? Clarified: it occurs after being hit, once per creature per turn.   You also gain advantage on saving throws vs. extreme heat and environmental fire hazards.   Level 18 — Avatar of the Forge   You become a living volcano on the battlefield.   Fire Immunity: You gain immunity to fire damage.   Volcanic Aura: As a bonus action, you erupt, creating a 15-foot radius aura for 1 minute:   Any creature that starts its turn in the aura or hits you with a melee attack takes fire damage equal to your Fighter level + proficiency bonus.   Creatures may make a Dexterity saving throw (DC = 8 + your Proficiency Bonus + your Strength modifier) to take half damage.   Once used, this feature can be used once per long rest.