Path of the Fiend

The Nine Hells and the Abyss are both profoundly evil, but in starkly contrasting ways. The Nine Hells wants to rule the world through subservience and law, all souls forfeit to the great Asmodeus. Where as the demon lords of the Abyss seek total annihilation through chaos, appeased only when the material plane is all but dust. You have become infused with the essence of these planes either thru a ritual bath in fiends’ blood, a wound from fiend that poisoned you with their nature, or maybe fiendish blood runs in your veins. Whatever the reason this conflict of law and chaos rages within you.

Fiendish Vigor

3rd-level Path of the Fiend feature

When you enter a rage you undergo a dark transformation, your eyes turn black and horns start to protrude from your head if they don't already. Until your rage ends you manifest power from either the lawful devils or the chaotic demons, choose one of the following options each time you rage:

Law. Shadows writhe around you orderly coalescing onto your weapons, extending their reach to 10 feet and granting them the magical property.     

Chaos. Chaotic energy erupts from you threatening to tear you and others apart. While raging the first creature you hit on each of your turns with a weapon attack takes an extra 1d6 + half your barbarian level force damage. If you miss all of your attacks on a turn you instead take the extra damage.

Aspect of the Lower Planes

6th-level Path of the Fiend feature

The Fiendish energies that dwell inside you have grown. When you choose Law or Chaos while raging you additionally gain the following benefits: 

Law. You see through both darkness and magical darkness out to 120 ft. The shadows around you now organize themselves against your others. The area within 10 ft of you becomes difficult terrain. 

Chaos. The temperament of demons invigorates your body. You gain temporary hit points equal to your barbarian level when you enter your rage.

Fiend’s Resistance

10th-level Path of the Fiend feature

The essences that reside within you dampen the effects of magic. You have advantage on saving throws against spells and other magical effects, and resistance to fire damage.

Fiendish Prowess

14th-level Path of the Fiend feature

You've gained complete control over the dark powers you have acquired. When you choose Law or Chaos when raging you additionally gain the following benefits: 

Law. The devil’s power inside of you has unlocked the ability to control magic to some extent. You can make a ranged spell attack akin to hurling a ball of flame at a single target. This attack does 5d6 fire damage, you are proficient and the attack is treated as a thrown object with a range of 60 ft.

Additionally, you can cast darkness once per rage.

Chaos. The chaotic energies inside of you cause your place in the physical world you be uncertain. When you take the attack action you can forgo one weapon attack to teleport 60 feet to a location you can see.