Barbarians that follow the path of the demon in one way or another have their soul bound to a demon. Some are the products of abyssal cults, some have been possessed in some way that still allows them agency, no mater how they got their demon their rage is fueled by the demonic strength and abyssal anger that persists in the reassesses of their soul.

Demonic Essence

Starting at 3rd level when you take this path your soul is intertwined with a demon, and it shows when you enter a rage. While raging you only speak abyssal (regardless of whether or not you know it normally), your veins turn black as they flow with ichor, any small plants within 5 feet of you wilt and die, and small beasts in the same vicinity are afflicted with fear.

Abyssal Resistance

Starting at level 3 your demonic essence protects you from spell effects. While raging you have advantage on saving throws against spells and other magical effects.

Demonic Rage

Starting at level 6 your inner demon can manifest during your rage to bolster your strength. Once per rage you can use a bonus action to increase your strength score by half of your barbarian level (rounded down) until the start of your next turn. This ability can go past your strength score maximum for its duration.

 

Ingrained Abyss

At level 10 your demonic essence has been growing in power and influence. Your Abyssal Resistance stays active even when not enraged.

Harden Ichor

At 14th level the black ichor that flows through your veins when you rage can pool and harden to reduce incoming damage. While raging, as a reaction to being hit by a physical attack that did not have advantage, you can reduce the damage by 1d10 + your constitution modifier + your barbarian level. If you would reduce the damage to 0 reduce it to 1 instead.