Fiendish path

This is the path for barbarians whose inner demons are a bit more literal, maybe a fiend made experiments on you with the goal of creating powerful soldier that couldn’t  be banished from the material plane, perhaps your tribe infused you with this power in a time of desperation, and in some rare cases you were a member of a divine order who voluntereed to become a vessel to seal a dangerous evil in you.

How is your relationship to this power is up to you and your DM, maybe the power is all your own, due to some fiendish pact or ancestry, pherhaps the power manifests as a shadow of your psyche, embodiyng all of negative aspescts that you see in you, or if you prefeer it could simply be a fiend, anywhere in betwenn an imp and a balor (whether it's a devil or demon is up to you) who has been sealed into you by magic, and keeps you alive either because your death would lead to their demise, or because they don't want to go back to their lower planes, again, this can have the complexity of a warlock patron or the simplicity of a barbarian anger issues, all up to you and your dm.

 

By the way in case you didn't figure this does have some "anime" vibes, like I try to keep it balanced, and themed in a way it doesn't break away from most dnd settings, but in case you, or your group don't like any sort of weeb stuff, well, you might wanna skip this one, sorry.

Fiendish Aura

Starting at level 3 you can tap into the forces within you in times of need to bolster your vitality, creating a Fiendish Aura, when you rage you can choose to gain temporary HP equal to the difference between your Maximum Hit point and current Hit Points, the temporary HP last as long as your rage does. Once you gained temporary hit points from this way you can't do so again until you finnish either a short or long rest.

Fiendish Strike

Beginning at level 6 whenever you use your reckless attack ability you can chose to make your attacks fiendish strikes, roll 1d4 to determine the damage type (1-Poison, 2-Cold, 3-Lightning, 4-fire), then you take 2d6 damage of the type determined by the roll, and whenever you make a damage roll with a weapon attack until the start of your next turn you add 2d6 to the roll of the damage type determined by the d4. The 2d6 you take and add to your rolls increases by 1d6 at 11th level (3d6), and 17th level (4d6).

Fiendish Resistance

Also at level 6 so long as you have at least one (1) temporary HP from your fiendish aura you gain resistance to acid, cold, lightning and fire damage.

Improved Fiendish Strike and Resistance

 Starting at level 10 when using fiendish strike you now roll 1d6 instead of the 1d4 to determine the damage type, following this table (1-Poison, 2-Cold, 3-Lightning, 4-fire, 5-Necrotic, 6-Choose one of the above), and your fiendish resistance also grants resistance against necrotic damage.

Fiendish Transformation

Starting at level 14 while raging you transform into a fiendish version of yourself, gaining leather bat wings, horns and other fiendish features, you gain a flying speed of 40 feet, if you your current hit points are less than half of your max hit points (not counting temporary hit points) you double that speed and can make one unarmed attack, dealing 1d6 piercing plus your strength modifier damage, as a bonus action, you chose the damage between piercing for your horns, slashing for your whip like tail or claws (you need not have a free hand for this unarmed attack).

ok, maybe 80 feet flying speed on a barbarian is too much... also the whole flight being restricted by the need to rage is going to end up creating few problems, since a lot of player will want to use the flying outside of combat and you don't get persistent rage until level 15... odds are they are just going to buy an alchemist fire, which, to be fair would make a really cool looking scene, in the mind theather that is, please don't light your mini's on fire...