Path of the Hidden Fiend

Barbarians which follow this path are mostly consisted of the ones who turn away from gods and let their souls get corrupted. Which is why Hidden Fiend barbarians are very rarely any kind of good alignment, if ever. When a barbarian using Hidden Fiend path rages, he looks into the darkest corners of himself and asks dark spirits for aid. Which mostly likely means some kind of bargain or deal with those spirits.

While raging, these barbarians forget bits and parts of their humane side and takes on a more demonic look. You could roll a dice or choose from the list, to add some cosmetic spices for your rage looks. 1D6 for:

1. Your eyes turn black and you have a slight black smoke coming from you.
2. You grow devilish spikes on your back, shoulders and hands.
3. Your body gets covered in multiple scars. 
4. Your face becomes pale and emotionless, with eyes very wide open.
5. Your veins become different colored and clear to see.
6. Your skin looks burnt and charred.

 

 

 

Nightmarish Swings

Starting from level 3 while raging, your reckless attacks channels your fiendish aggression through your weapon. On a successful melee reckless attack, once per turn, you can deal additional psychic damage equal to 1D4 + half of your barbarian level (minimum 1).

If the target is frightened of you, you instead deal psychic damage equal to 1D4 + your barbarian level. 

Devils true Form

After reaching level 3 in this path, you can cause fair among nearby hostile creatures. Whenever entering rage, all enemies 5 feet around you must make a wisdom save or become frightened of you until the start of your next turn. 

The required saving throw DC for this is 8 + proficiency bonus + your constitution modifier. Creatures with size small or below gets disadvantage on this save, while creatures of size large or above gets advantage.

Fiendish Executioner

When you reach lvl 6 in this path, you learn that while dealing a final blow to the enemy, with a melee attack, you can inflict fear upon other creatures around him. Any creatures hostile to you and located 10 feet around the enemy you slain, has to make a wisdom saving throw or be frightened of you until the end of your next turn. The range of this feat increases with level up. To 20 feet at 10th level and to 30 feet at 14th level.

Saving throw for this is 8 + proficiency modifier + your strength modifier. Creatures of size small or below gets disadvantage on this save, while creatures of size large or above gets advantage. This effect cannot stack.

Morale Trampler

As mentioned before after reaching level 10, Fiendish Executioner gets increased range of 20 ft around slain creature. Additionally you get even greater advantage against enemies which are afraid of you. Any creatures who are frightened of you cannot make opportunity attacks against you.

Fearless One

After reaching level 14, You become the epitome of fear. Fiendish Executioner  gets increased range of 30 ft around slain creature. And as a master of battlefield control you get immunity to being frightened or charmed while you are enraged.