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.

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 darkvision. 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 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.

The Completed Pact

At 14th level you have fully mastered and tamed your demonic powers. You are now immune to fire damage and can cast the cantrip [Tooltip Not Found] (wisdom is your spellcasting). Once per week for 10 minutes, you may conduct a blood ritual. To perform the ritual you must need 15 cubic feet, arcane chalk, a vial of humanoid blood (not of your own), a vial of the blood of a beast, and a vial of demon blood. You may decide how the ritual is conducted with the components. During the ritual, although you must remain motionless or else the ritual will fail, if you are attacked and take damage roll a DC 16  Constitution saving throw if you fail the ritual ends if you succeed the ritual continues and you remain stationary. Once the ritual is completed a 15-foot crack opens in the ground with the flames of the underworld spewing out. Roll a d100 on the demon summoning chart. Any fiend summoned this way serves you with complete loyalty for 1 month or until they die, after that, they return to whence they came. You can speak to these servants telepathically for up to 1 mile. You can only have up to 3 fiend servants at once. 

Fiend Summoning Table

d100	Summoned Fiend
01-23	Maw Demon
24-41	Hell Hound
42-58	Bulezau
59-73	Shadow Demon
74-85	Vrock
86-93	Glabrezu
94-97	Nalfeshnee
99-100	Nabassu