Path of the Firestorm Berserker

A Firestorm Berserker personifies the inferno. When they rage, mind and spirit are engulfed in a blazing uncontrolled fire that feeds on everything. Their inner self is like dry kindling, ready to erupt into flames at any moment. In the beginning, the fire of their rage burns only within, showing only small outward signs of its presence. With experience and greater and more ferocious battles, the flames of their fury manifest in conflagrant destruction of all things around them. Every Firestorm Berserker faces a constant battle with their own inner blaze to keep control, lest the flames consume them, mind and body. 

Burning Rage

At 3rd level, you can unleash the fire within into a raging inferno when you rage. While raging, you can choose to enter a burning rage. In burning rage, you have resistance to fire damage and deal an additional 1d6 fire damage with all your strikes. At the end of each of your turns while in burning rage, you must make a DC 10 constitution saving throw. On a success, you can choose to end your burning rage and return to normal rage. On a failure, you suffer one level of exhaustion, the DC of the next save increases by 2, you remain in burning rage, and you must use any remaining movement to move toward the nearest living thing, and if you get in range, you must make an attack. Levels of exhaustion gained this way do not take effect until after your rage ends. If you reach six levels of exhaustion, your rage ends immediately and you drop to 0 hit points. If you die as a result of this effect, your body crumbles to ashes. 

Kindling

At 3rd level, the slightest spark can ignite the fire within. You can go into a rage as a reaction rather than a bonus action.

Breath of Fire

Starting at 6th level, you can concentrate your soul's fury into a fiery breath. While in your burning rage, you can use an action to breathe fire in a 10-foot cone. Each creature in this area must make a DEX saving throw, taking 3d6 fire damage on a failed save and half as much damage on a successful one. After you do this, your burning rage ends, returning you to normal rage. Your burning rage now gives you resistance to cold and your strikes deal an additional 1d8 fire damage.

Sheathed in Flame

At 10th level, you soul has firmly stoked its embers, and your flesh erupts in flames with the power of your rage. You gain resistance to fire damage even when not raging. While in your burning rage, you have immunity to fire damage and resistance to cold damage, and your strikes deal an additional 1d10 fire damage. When you attack recklessly in burning rage, your strikes deal extra fire damage equal to your constitution modifier.  While in your burning rage, any creature within 5 ft. of you that hits you with a melee attack instantly suffers 1d8 + your constitution modifier in fire damage. 

Venting Blast

At 14th level, while in your burning rage, you can release all your inner flaming fury in one great shout. All creatures within a 15-foot radius of you must make a DEX saving throw, taking 8d6 fire damage on a failed save or half as much on a damage on a successful one. After doing this, your burning rage and rage end immediately, you take one level of exhaustion, and cannot do anything until your next turn. You now have resistance to cold damage even while not raging. While in burning rage, you have immunity to fire and cold damage, and  your strikes deal an extra 1d12 of fire damage. While in burning rage, whenever a spell with a range other than self or touch deals fire damage to a creature within a 30-foot radius of you, the fire damage goes to you instead and you gain half the fire damage that would've been dealt as temporary hit points that last until the end of your next turn. If you score a critical hit while attacking recklessly in your burning rage, all damage from your brutal critical dice becomes unresisted fire damage, and you can spend any temporary hit points you have to deal that many points of fire damage to a creature within 30 feet of you in your line of sight.