Possessed Soul

Barbarians are simple people, so they are easy to possess, and a spirit of anger has taken hold of this specific one. This spirit is a dormant one, only taking hold when the barbarians emotions get too strong, causing the spirit to take control for a short amount of time. The spirit grants the barbarian unnatural and dark powers.

Dark spirit

The spirit that possesses the barbarian follows them as a permanent imp familiar. While raging, the familiar takes hold of the barbarians body and the imp familiar disappears.

Unnatural powers

The Imp familiar that follows the barbarian around can cast Eldritch blast. Only while raging, the barbarian can cast this spell. While in control, the dark spirit grants the barbarian enhanced and dark senses. (120 ft dark vision) While not in control, these senses linger, although less powerful. (60 ft dark vision)

Unbothered

After some time, the barbarian gets used to being possessed, they grow almost comfortable with it. Because of this, they cooperate with the spirit.

Demonic savagery

The spirit helps the barbarian by cursing any weapon they hold

Demonic skin

Unnaturally hardens the skin of the barbarian.

Give in to the darkness

The barbarians skin darkens, their eyes gain a red glow, their nails grow sharp, and their affinity with the dark arts grow greater. The barbarian can now cast eldritch blast while not raging.

Demonic soul

The demon and the barbarian are separate but completely tied to each other. The familiar no longer disappears when the barbarian rages.