Barbarians travel far and wide taking up quests for great weapons of power or perilous contracts to fill in order to earn their fame. Some barbarians look only to themselves, their companions, or their clans for strength. Others still are inspired not by the men they face in life, but the monsters. Barbarians who follow the Path of the Fiend are those who revere the strength of otherworldly demons, so much so that they are willing to become like one. They may worship one of the demon lords, they may worship themselves, or they may only share that undeniable thirst for domination. (Adventurers' Appendices pg.3) 

Fiendish Nature

At 3rd level when you adopt this path your admiration for the Abyss begins to draw you in and transform your nature. You become resistant to poison damage, are now proficient with the intimidation skill, and are faintly recognized as fiendish by the spell detect good and evil.

Variant Rule: Demonic Boon

If your DM allows it, you may select one demonic boon from the Demonic Boons section beginning on pg.30 of Mordenkainen’s Tome of Foes.

Demonic Form

Starting when you choose this path at 3rd level, your body begins to transform into one of the demons you so admire. You start to develop any combination of either cloven feet, elongated claws, or horns. You gain the following benefits while you are either unarmed or are wielding only a single one-handed weapon:

     You can roll a d6 in place of the normal damage of your unarmed strike. This die changes as you gain barbarian levels. Your unarmed attacks grow to a d8 at level 7, a d10 at level 11, and a d12 at level 15.

     When you use the Attack action with an unarmed strike or a one-handed weapon on your turn, you can make one unarmed strike as a bonus action, assuming you haven't already taken a bonus action this turn.

Charge of the Armanite

Beginning at 6th level, you can harness your rage into discharges of violent energy. Once per short rest you can take a special dash action while raging, and any creature you choose within 10 feet of your path becomes the target of a discharge and must make a DC 15 Dexterity saving throw. Each creature takes a number of d8 lightning damage equal to your Strength modifier, or half as much on a success.

Hunger of the Devourer

At 10th level you learn how to hunt living souls as a demon. A number of times per day equal to your Constitution modifier you can release a wave of negative energy as a bonus action. Any living creature within 60 feet of you has its location revealed to you, and if a creature is currently grappled by you when you use this ability it takes 2d12 necrotic damage and heals you for half that amount. This ability has no effect on constructs or undead.

Grip of the Balor

Starting at 14th level, your command of demonic energy grows highly concentrated. When entering rage you choose either lightning or fire damage. For the duration of that rage you are resistant to the damage type you chose, and your melee attacks deal an extra d8 of the same damage type. While under this effect, your unarmed strikes are considered magical for the purpose of overcoming physical resistances.