Oni Touched

Barbarians who are Oni Touched draw their rage from a Demon Burning within their souls. That Oni bursts forth in the throes of rage, physically transforming the barbarian.

Such a barbarian might be inhabited by a Demon Spirit or gained it from ancestors.

The Oni

3rd-level The Oni feature

When you enter your rage, you transform, revealing the Demon power within you. Until the rage ends, you are transformed into the Oni. It counts as another Form with its own HP and Attacks, and it  has a Strength of 19  (modifier +4) to the attack and damage rolls when you attack with it.
Rest of the Modifiers are Yours
Also you gain proficiency in intimidation.
When the Oni drops to 0 HP you revert to your normal self and the oni needs a short rest to fully heal up.
If the oni is still alive it only needs a minute per 50 HP.

The oni is a Large Creature, however if there is not enough hight for the Oni to form, it will simply grow to the hight it can, Still considered a Large creature. (This can only happen if you transform where you can stand, No tiny oni's here)

The Oni Stats.
Large Demon (minimum 7 feet) 

Armor Class: 13+constitution modifier
HP: 1D10 (or 6) + your Constitution modifier  per Barbarian level +39
Speed 30 ft.

Feats:
Magic Weapons. The oni's weapon attacks are magical.
Regeneration. The oni regains constitution modifier + proficiency hit points at the start of its turn if it has at least 1 hit point.

3 Attacks per action

Oni Club. Wielding your massive club to smack someone.
Melee Weapon Attack: +4 (+proficiency) to hit, reach 10 ft., one target. Hit: 2d10 + 4) Bludgeoning damage.

Smash: Hitting your target and any creature surounding it
Melee Weapon Attack: +4 (+proficiency) to hit, reach 10 ft., one target. Hit: 1d10 + 4) Bludgeoning damage, Half damage to all adjecent creatures.

Bellowing Roar: One Roar to put fear into your enemies.
any creature within 20 feet of you needs to make a Wisdom save (against your 10+constitution modifier + proficiency)  if they fail, they are Paralized.


_______________
Special:
When you transform back and the oni still has Hp left, it will be fully healed in 30 minutes,  If it has died than it needs a short rest to come back.
The ony does not gain

• resistance to bludgeoning, piercing, and slashing damage.

From your normal rage. It does gain the other atributes.

• advantage on Strength checks and Strength saving throws.
• When you make a melee weapon attack using Strength, you gain a bonus to the damage roll that increases as you gain levels as a barbarian, as shown in the Rage Damage column of the Barbarian table.

 

Blood Gluttony

6th-level Oni touched feature

The blood cravings of your Oni will allow you to dash to your next target once one has been brought to their knees.
When you bring a creature to 0 HP you can use your reaction to move to another within your movement range.

Demonic Infection

10th-level Oni Touched feature

When you hit a creature with your Oni weapons, the Oni can Possess the target. The target must succeed on a Wisdom saving throw (DC equal to 8 + your Constitution modifier + your proficiency bonus) or suffer one of the following effects (your choice):

• The target must use its reaction to make a melee attack against another creature of your choice that you can see.
• The target takes 2d12 psychic damage.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

True Yokai

14th-level Oni Touched feature

While you’re in Oni Form, having 0 hit points doesn’t knock you unconscious. You still must make death saving throws, and you suffer the normal effects of taking damage while at 0 hit points. However, if you would die due to failing death saving throws, you don’t die until your rage ends, than you Revert to your normal self with 1 HP. The Oni than needs to reheal with a short rest.

While below 0 HP you can roll 1 extra Damage die from the Oni Weapon Attacks.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.