Supreme Path of the inner demon

A Barbarians rage is fueled either by joy, anguish, or hatred. But these barbarians draw their strength from fiendish origin, weather forced by or influenced by theses demonic forces these barbarians burn with the flames of their rage and devour the blood of those they slaughter. So the true question is weather they're still man or a monster. 

These Barbarians have made some sort of pact with a Fiend (demon or devil) and now draw their power from them, sometimes being used as a tool by the fiend. The barbarian may break free from this pact some how and may then choose to remain with this Path or switch to another.

 

Among the darkest and most savage tribes of the wilds, demons supplant the gods and spirits of the old faiths. The demons turn these tribes from harmony to conquest, and goad them into violence against their neighbors. In other cases, pushed to desperation by a few seasons of scarce game or the pressures of stronger neighbors, a tribe may seek out the patronage of a demon. In exchange for their service, these demonic patrons grant extraordinary power. Just as the totemic barbarians gain traits of beasts, the demonscarred gain traits from the Abyssal legions. There are also those who simply lived too long in places tainted by the power of the Abyss, and found that the totemic spirits of the earth could no longer bear their presence; such individuals make the best of a difficult situation and often wield their powers against the Abyss.

 

The DC for abilities granted by this Path that require a saving throw is 8 + your Constitution bonus + your proficiency bonus.


Starting when you choose this path at 3rd level the demonic influence on your rage damages and warps your physical form.
Your inner darkness begins to manifest visibly when you enter a rage. 
While raging, your appearance changes in subtle ways (your eyes turn solid black or red, your teeth and horns become sharpened and pointed. 
This change becomes more and more pronounced as you gain Barbarian levels (horns lengthen, skin becomes milky white or takes on reddish hues, face becomes increasingly fiendish). You gain advantage on Charisma (Intimidation) checks and may add your Strength modifier instead of your Charisma modifier to these checks. However this change also imposes disadvantage on your Charisma (Persuasion) checks.

 

Blood of the Fallen

At 3rd level while raging, when you kill any creature you can spend a bonus action absorbing the blood restoring d6 hitpoints. Due to your fiendish power, you are unable to use Blood of the Fallen if you took radiant damage on the same round.

At higher levels

At 6th level when using Blood of the Fallen, regain a d8 and 2d8 at the 10th level

Hellish Heart

At 3rd level you can speak abyssal and are resistant to fire damage and can cast the spell Detect Evil and Good once per long rest. When in darkness your eyes glow red as you gain 60 feet of dark-vision which combines with any existing dark-vision. The demonic power within you comes with a grave toll, you gain 3rd level exhaustion when in holy temples/grounds, 1 minute after leaving holy temples/grounds you lose the exhaustion effects.

Devilish Luck

At 6th level when raging 19 and 20 are critical's and if you kill a creature with a critical gain an additional attack action.

Demonic Possession

Starting at 6th Level your rage pulls your conscious mind away from your body and opens you up as a vessel for a lesser demon. when you enter a rage the dm rolls a d20. On a 5 or lower your body is possessed by a demon giving you the following effects:

• You gain an additional 10 feet of movement speed.
• You gain resistance to Fire and Necrotic Damage.
• You attack the creature nearest you (allies included). If there is more than one creature equally near to you choose one at random.
• If no creature is within your melee range you move towards the nearest one (allies included). If more than one creature is equally distant from you choose one at random.

Demonic Fury

At 10th level when raging hellish flames begin to resonate from your body, when a creature deals melee damage to you it takes 2d6 of fire damage and all melee attacks you make deal an additional d8 of fire damage. While raging you can use an action to create a flaming hellish spectral weapon in your hands, the weapon can be any simple weapon and uses the stats of such weapon.

Intimidating Presence

Beginning at 10th level, you can use your action to frighten someone with your menacing presence. When you do so, choose one creature that you can see within 30 feet of you. If the creature can see or hear you, it must succeed on a Wisdom saving throw (DC equal to 8 + your proficiency bonus + your Charisma modifier) or be frightened of you until the end of your next turn. On subsequent turns, you can use your action to extend the duration of this effect on the frightened creature until the end of your next turn. This effect ends if the creature ends its turn out of line of sight or more than 60 feet away from you.

If the creature succeeds on its saving throw, you can’t use this feature on that creature again for 24 hours.

Slaughter

Starting at 10th level death fuels and empowers you. While raging, whenever you reduce a creature to 0 hit points with a melee weapon attack you gain a +1 to attack and damage rolls. this bonus continues to stack until your rage ends at which point you lose them.

Pact with the Devil

When you reach 14th level, you no longer age, can't be aged
magically, and can't die of old age. The spells imprisonment
and seal are added to the sorcerer spell list for you. If you are
ever under the effects of the imprisonment spell, you can
choose to end it using your action.
Finally, you can release fraction of your horror’s power,
damaging all who deserve it.

You utter foul words, summoning one demon from the chaos of the Abyss. You choose the demon’s type, which must be the "Pact Devil". The demon appears in an unoccupied space you can see within range, and the demon disappears when it drops to 0 hit points or when the spell ends.
Roll initiative for the demon, which has its own turns. When you summon it and on each of your turns thereafter, you can issue a verbal command to it (requiring no action on your part), telling it what it must do on its next turn. If you issue no command, it spends its turn attacking any creature within reach that has attacked it.
At the end of each of the demon’s turns, it makes a Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name. On a failed save, the demon continues to obey you. On a successful save, your control of the demon ends for the rest of the duration, and the demon spends its turns pursuing and attacking the nearest non-demons to the best of its ability. If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn’t disappear for 1d6 rounds if it still has hit points.
As part of casting the spell, you can form a circle on the ground with the blood used as a material component. The circle is large enough to encompass your space. While the spell lasts, the summoned demon can’t cross the circle or harm it, and it can’t target anyone within it. Using the material component in this manner consumes it when the spell ends.