Immolationist

Some fighters strike with their strength alone, some are backed by magic, others by echoes of themselves, and still others have learned that true firepower stems from fire itself. They form groups near volcanoes and lava pools. If you see a forest fire, there's a chance that it was started by an Immolationist. They are fire. They are heat. They are Immolationists.

(Credit to Lord_Light for making this)

Fire-Wrought Body

At 3rd level, flames have seared your skin, providing your body with enhanced protection. You gain resistance to fire damage, your fire damage now bypasses resistance, and you are immune to the effects of extreme cold and heat.

Unleash Hell

Additionally at 3rd level, you may, as a bonus action on your turn, coat your weapon in flames for one minute, causing it do deal an additional 1d4 fire damage on a hit. This damage increases to 1d6 at 10th level, and 1d8 at 18th level. You may use this a number of times equal to your proficiency bonus, regaining all uses at the end of a long rest.

Thermodynamics

Beginning at 7th level, as a reaction to taking fire damage, you can absorb part of the damage. When you use this reaction, your next attack that does fire damage deals additional fire damage equal to half the damage that you took. You can only store one instance of damage at a time. You may use this a number of times equal to your constitution modifier, regaining its use after a long rest.

Burn Bright. Burn Hot.

Starting at 10th level, your flames burn brighter than the sun. As an action you can channel your inner flames and unleash a fiery aura. When activated, choose one of the following effects:

- You release a sudden flash of light. All creatures within 15 feet of you must make a Dexterity saving throw. Allies have advantage on the save. On a failure the creature is blinded for 1 minute, the creature can repeat this save at the start of their turn to end this effect early.
- You release a steady stream of fire, as if you were concentrating on a spell. For the next minute, targets of your choice within 15 feet of you must make a Constitution saving throw. On a failure the target takes 2d6 fire damage, and half as much on a successful one.

The DC for these effects is 8 + your proficiency bonus + your constitution modifier. You regain use of this feature after a short or long rest.

Flame of Life

Beginning at 15th level, whenever you use your Second Wind feature, you roll an additional 1d10 for healing.

Additionally Your fire damage now treats immunity as resistance. However, this feature does not work on elementals or creatures that can be healed by fire damage.

Rising Phoenix

At 18th level, your flames not only bring forth destruction, but are surging with life. When you are reduced to 0 hp, you can instead choose to gain back hit points equal to one-third of your max hp. You can use this feature once per long rest.