Path of the Damned

Some barbarians are not the product of any particular school of martial training or the harnessing and direction of natural rage or battles trances, but of supernatural possession. These unfortunates struggle endlessly against a literal demon within for conquest of their soul. For a resolute or lucky few, they've made a sort of relative, uneasy "peace" with their dark passenger, achieving a perverse symbiosis. In exchange for a measure of the demon's power, the host covenants to indulge its bloodlust and fury of their capacity as a warrior; conflicts their abyssal stowaway is sure to encourage and escalate provided a fraction if the opportunity.

The Demon Within

Starting at 3rd level, your rage brings the demon within to the fore, enacting a partial transformation of your person. You learn the Abyssal language and gain proficiency in the Intimidation skill. Whenever you rage, you can gain up to two of the following benefits until your rage ends:

Demon Resilience. You gain resistance to cold, fire, and lightning damage.
Magic Resilience. You have advantage on saving throws against spells and other magical effects.
Poison Immunity. You gain immunity to poison damage and the poisoned condition.
Abyssal Sight. You gain darkvision out to 120 feet.

By Demons Be Driven

At 6th level, the spilling of blood excites and empowers the fiend within, allowing it a small measure of control. Whenever you reduce a creature to 0 hit points while raging, make a DC 12 Charisma saving throw. On a success, you can use your reaction to immediately move up to your speed to a space adjacent to a creature you can hear or see, then make a weapon attack against it. On a failure, you must use your reaction in this way if possible.

Abyssal Bloodlust

At 10th level, your demon seizes upon your blind fury to goad you into greater heights of violence and reckless carnage. Whenever you hit with an attack that benefits from your Reckless Attack feature while raging, you can expend a Hit Dice. If you choose to do that, or if the attack critically hits, you can use your bonus action immediately after making that attack to make a melee weapon attack that deals 1d12 extra force damage.

The Demon Without

At 14th level, the demon inside has grown strong, manifesting its influence whenever you indulge your boundless wrath. Whenever you rage, you must make a DC 15 Charisma saving throw. On a success, you can magically transform into a demon until your rage ends. Otherwise, you must transform if possible, and you can't willingly end your rage for 1 minute. While transformed in this way you count as being Chaotic Evil (your alignment doesn't change) for the purposes of all effects, and your creature type is fiend(demon). Furthermore, if you failed this feature's Charisma saving throw, you have all of the following benefits, otherwise you can have any of them:

Abyssal Wings. Wings of abyssal fire sprout from your back, giving you a flying speed equal to your walking speed.
Fiendish Bulk. Your size and the size of your worn and carried equipment increase as per the Enlarge effect of the enlarge/reduce spell.
Vile Metamorphosis. You gain the benefits of all the options of your The Demon Within feature, and you gain advantage on Charisma (Intimidation) checks.

Whenever you end your turn without making a melee weapon attack against a creature during that turn while you can see a hostile creature, you take 2d12 psychic damage that ignores resistance and immunity.