Path of the fiend

Through a contract with a devil, or infernal heritage your fury rakes the form of a fiend, transforming your body into that of a hellish creature while you rage

fiendish

Fiendish. 3rd level. You can now read, speak, and write in both abyssal and infernal. You also have advantage on intimidation checks against non fiend creatures.

 

hellish strikes

Hellish Strike. 3rd level. While you're raging, the first creature you hit on each of your turns with a weapon attack takes extra damage equal to 1d6 + half your barbarian level. The extra damage is fire or poison your choice each time you hit.

fiends body

Fiend's Body. 6th level. When you take damage while raging, you can deal your constitution modifier in fire damage, as a reaction. Also while you rage you gain resistance to fire damage. If you already have fire resistance then you can chose to gain poison or cold resistance instead. At 14th level the resistance becomes immunity.

corrupt power

 

Corrupt Power. 10th level. When you hit a creature with your hellish strike or Fiend's body you gain 1 charge of corrupt power. You can use your corrupt power in one of the following ways:

Inferno. As an action you can target a creature or space within 30 feet. Creatures and objects within 10 feet of the target, including the target, take a number of d6s of damage equal to the expended corrupt power charges. The damage is either fire or poison your choice.

Contract. As a bonus action you can empower an ally within 60 feet. Your ally gains 5 temporary hit points for each corrupt charge spent.

You can have a maximum of half your barbarian level in corrupt power charges. At 14th level your inferno ignores resistance and treats immunity as resistance.

 

soul siphon

 

Soul Siphon. 14th level. For each sentient creature that dies within 30 feet of you you gain +1 to damage rolls. At the end of a short or long rest you lose this bonus damage. You can have a number of soul shards equal to your proficiency bonus. A creature must be hostile toward you to gain the soul shard.