Possessed Fighter

You have been forged through years of great suffering and an inheritance of cursed blood. A mad, raging spirit now owns your body, and as you die, a thirst for blood consumes you. You are transformed into a living nightmare.
when possessed, can only use unarmed strikes and can absorb part of the enemy's health with each hit

Awakening

When you choose this archetype at 3rd level, you grow claws, fangs, spines, horns, or a different natural weapon of your choice. Your unarmed strikes deal (STR) 1d6 bludgeoning, piercing, or slashing damage, as appropriate to the natural weapon you chose, and you are proficient with your unarmed strikes. Finally, the natural weapon is magic and you have a +1 bonus to the attack and damage rolls you make using it.

• And by reaching level 7, your unarmed strikes deals  1d10 +2.
• And by reaching level 10, your unarmed strikes deals 3d6 +4.
• And by reaching level 17, your unarmed strikes deals 4d8 +7.

Soul Devourer

Beginning at 7th level, you recover 1 hit point for each successful attack, and 2 hit points if it's a critical hit, also you add a +2 bonus to your damage roll.
And if you drop bellow 10 Hit Points, you turn into a Nightmare Demon and double the hit points you absorb for normal and critical attacks until you recover above 20 Hit Points.
Also you can push a Foe away as a Bonus Action once per turn dealing 1d6 bludgeoning damage until the Nightmare form lasts.

(You can only use unarmed strikes while possesed).

• At level 10, the Nightmare Possession begins by dropping bellow 20HP and lasts until you recover above 30 Hit Points.
  (you recover 3 hit points for each successful attack, and 10 hit points if it's a critical hit).
  
  
• At level 18, the Nightmare Possession begins by dropping bellow 45HP and lasts until you recover above 70 Hit Points.
  (you recover 6 hit points for each successful attack, and 20 hit points if it's a critical hit).
  

Refuse Death

At 10th level, you always become resistant to Acid, Poison, Fire, Cold, Lightning, Necrotic, Thunder, Radiant, Bludgeoning, Piercing, and Slashing type damage when transformed in Nightmare form. This resistance lasts until you recover from nightmare state. (unless you are unconscious).

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At 10th level:
If you drop to around 1 - 20HP,  you instantly receive a reaction attack, you pierce your natural weapon true your foe's heart and deals 1d20+(modifiers) as Necrotic damage and recovers all damage received in the last blow
(you can use this Reaction once per short rest.)

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At 18th level:
If you drop to around 1 - 40HP,  you instantly receive a reaction attack, you pierce your natural weapon true your foe's heart and deals 3d20+(modifiers) as Necrotic damage and recovers all damage received in the last blow
(you can use this Reaction once per short rest.)

Abomination

At 15th level you gain additional Hit Points equal to your constitution modifier + 1/3 of your actual HP.
And when you enter Nightmare State, shadows from the souls you killed starts flying around you raging and forming a dark cloud of spirits that deals 1d10 of Slashing Necrotic Damage to everything (except you) in your melee range at every turn and obstructs the vision of every creature outside the dark blaming cloud, But you have you movement action halved while the cloud is up.

These shadows go away when you recover from your Possession.

Battle cry

At 18th level, if an Ally is injured about less than half his Total HP, you can shout to the enemy to provoque and force all enemies up to 10ft to focus their attacks on you.
Then Roll 1d20+(Intimidation)+(proficiency bonus)  DC:15.
If successful, you gain +50 Temporary Hit Points (these can't be healed) for each enemy focusing you. This Provocation lasts for three rounds.

(And all of your normal attacks Recovers 10 Hit Points. And 20 Hit Points if its a critical hit until this effect lasts.)