Fireborn

All fighters know that their skill is only a fraction of some ancient power that rampages and kills indiscriminately. Many give themselves just enough control to prevent themselves from giving in to this power. But sometimes, some souls decide to meet this power on their own terms, attempting to summon this power at risk of destroying their bodies. Sometimes such a challenge happens subconsciously, and sometimes the creature does it willingly. Those that learn to embrace this forbidden power, that learn to make it feed on their rage, gain inconceivable strength and fury, borne of the flame.

Burning Attacks

Starting at 3rd level, as the fury of the flame begins to burn within you, you learn to imbue your attacks with the heat of battle. As a bonus action, you empower yourself with fire. All of your attacks deal an additional amount of fire damage equal to a roll of your Fire Die for 1 hour, and you gain resistance to fire and poison damage for the duration. You can use this empowerment a number of times equal to your proficiency bonus, and regain all expended uses on a long rest. Your Fire Die start as d4s, turn into d6s at 7th level, turn into d8s at 10th level, and turn into d10s at 15th level.

Heart of the Fire

Starting from 3rd level, the fury burning in your blood allows your wounds to mend quickly. When you use your Second Wind feature, you can spend a number of Hit Dice up to your proficiency bonus, roll them, and regain additional hit points equal to the rolls + your Constitution modifier.

Vengeful Flames

Starting at 7th level, fiery tendrils lash out with anger at your opponents when you are knocked down. When you are reduced to 0 hit points, you choose a number of creatures within 15 feet of you up to your Constitution modifier. You make attack rolls toward all of them. On a hit, they take fire damage from a number of rolls of your Fire Die up to your Fighter level.

Firepower Mutation

Starting at 10th level, your vengeful gift warps within you, twisting it into a greater source of power. When you use your Burning Attacks feature, you can choose a new damage type: lightning or radiant. When you roll your Fire Die, you can add lightning or radiant damage to it equal to a new roll of a Power Die (which start as d6s, and turn into d8s at 14th level).

Power of the Light

Your Power Die type is radiant damage.

Power of the Storm

Your Power Die type is lightning damage.

Heat Resilience

Starting at 15th level, your affinity to fire grants you protection from the havoc it wreaks. You permanently gain resistance to fire damage. In addition, once during your Fire Empowerment, you may, as a bonus action, grant yourself immunity to fire damage for 1 minute.

Inferno

Starting at 18th level, your body blazes with elemental power and sparks swirl around you. While empowered, you emit a radius of 15 feet in which the ground burns. The radius is considered difficult terrain. Any creature that enters the radius for the first time or starts its turn there takes 5d10 fire damage as the flames strike it. You can designate a number of creatures up to your Constitution modifier to ignore these effects.