Path of the Hellborn

Some barbarians are captured by the raw power of destruction that is fire, they can’t get enough of it and seek to feel its sting to fuel them in battle and destroy their foes. Those warriors, either psychopathic lunatics and crazed firebugs looking to see the world burn, or the successors to an infernal heritage, stemming from a deal struck generations ago, still lurking in their souls, waiting to unleash itself and bring hell upon the world, they follow the path of the hellborn.

 

Also at 20th level you are consumed by the hellfire that burns in your soul. Your unlimited rage is replaced by an unlimited burning rage, whenever you go into a rage you go directly into a burning rage. The hellfire that fuels your rage can not be doused anymore by any means but your will or powerful magic like a Wish spell.

(It seems it is not possible to add class features at incorrect level in dnd Beyond, so I guess I'll add it here.)

Burning Rage / Burn, baby! Burn!

Starting when you choose this path at 3rd level, you can enter a burning rage (as a free action) while you are raging. If you do so, your skin erupts into flames, you take 1d6 points of fire damage every round and your attacks deal an additional 1d6 points of fire damage. Should the flames be doused, for example through immersion in a large body of water, your rage ends prematurely. Should you be immune to fire damage you are unable to harness the power of the flames, as your body refuses to ignite.

Fire Resistance / Fire at the Disco!

Beginning at 6th level, you embrace the kiss of the flame. You become resistant to fire damage while in burning rage. You also learn to harness the pure Hellfire in your soul, treating Fire Immunity against the damage you deal as Fire Resistance while you are in a burning rage.

Fire Fiend / Disco Inferno

Beginning at 10th level, you cannot hide anymore that your body is filled with the infernal hellfire that is burning in your soul. Your eyes burn away and are replaced by glowing orbs of molten lava and you always exude a noticeable odor of sulfur and fire.

Once per burning rage you can use your action to release a flaming inferno from your body. The DC for this ability is equal to 8 + your proficiency bonus + your strength modifier. For the purposes of damage and saving throw, treat this ability like the Fireball spell. However this is not a spell and as such cannot be dispelled or affected by any item or ability that affects spells or spell saving throws.

Hellish Eruption / Burn, Motherfucker! Burn!

Beginning at 14th level, when you reduce an enemy to zero hitpoints while in a burning rage, your hellfire infused attacks turn that creature into a deadly threat for friend and foe. In the next round after it was reduced to zero hitpoints (when it would act on its turn if it were still alive), the creature bursts in an explosion of Fire, blood and viscera and deals 4d6 points of fire damage to every creature in a 10 ft. radius around it. A successful Dexterity saving throw against a DC equal to 8 + your proficiency bonus + your strength modifier halves the damage taken.