Path of the Monster

This subclass may only be taken by those with the creatures types undead, monstrosity, aberration or fiend, or the Reborn and Dhampir race. Instead of embracing the life given, barbarians of this subclass throw away their humanity, choosing to become monsters of some ilk. This choice leads to them drawing on great powers, but the cost is great as well. After all, the greatest villains all have a grand weakness. Follows of this path would sacrifice much for this power, sometimes knowing what they lose in return. Sometimes not caring.  

Dark Pact

Starting when you choose this path at 3rd level, select a damage type (You cannot select homebrew damage types). While you are raging, you are vulnerable to that type of damage. In addition, you cannot gain resistance or immunity to this type, or reduce the damage it deals to you regardless of form. While raging, your features change to more obviously exemplify your inhuman nature, such as growing longer fangs, dripping blood, or changing your facial structure. Depending on what vulnerability you choose, your rage gains an additional property

Elemental (Acid, Cold, Fire, Lightning, or Thunder) - When you take damage while raging, The first time you hit with a melee attack on your next turn, it deals 1d6 additional damage of the type dealt (this can stack)

Physical - (Bludgeoning, Piercing and Slashing) - While raging, you can select an amount of hit points, to a maximum of your max hit points minus 10. You lose that amount of hit points, and gain temporary hit points equal to double the amount lost in this way. While you have these temporary hit points, attacks cannot have advantage against you, and you can expend a hit dice whenever you fail a saving throw, potentially turning a failure into a success. 

Unnatural (Psychic, Force, Radiant, Necrotic) - While raging, your natural weapon and unarmed attacks deal additional damage equal to half your barbarian level, rounded down, in d4s. This additional damage can be one of the types listed, except your vulnerability. 

Elemental

When you take damage while raging, The first time you hit with a melee attack on your next turn, it deals 1d6 additional damage of the type dealt (this can stack)

Physical

 While raging, you can select an amount of hit points, to a maximum of your max hit points minus 10. You lose that amount of hit points, and gain temporary hit points equal to double the amount lost in this way. While you have these temporary hit points, attacks cannot have advantage against you, and you can expend a hit dice whenever you fail a saving throw, potentially turning a failure into a success. 

Unnatural

While raging, your natural weapon and unarmed attacks deal additional damage equal to half your barbarian level, rounded down, in d4s. This additional damage can be one of the types listed, except your vulnerability. 

Sinister Mentality

Beginning at 6th level, even when raging, you use your wit, charm or intelligence to it's fullest advantage, along with your physique. You gain one of the following abilities, based on the highest ability score you have among Wisdom, Intelligence or Charisma (if that score changes, this ability also changes it's effect). 

Wisdom - You have disadvantage against saving throws versus the paralyzed condition, and you cannot gain immunity from that condition. You gain proficiency in Wisdom saving throws. You gain the ability to make a magical ranged attack, in place of a melee attack while raging. It deals the same amount of damage as the normal attack, but has a range of 120 feet, and uses your wisdom, instead of strength for the attack roll. 

Intelligence - You have disadvantage against saving throws versus the charmed condition, and you cannot gain immunity from that condition.  You gain proficiency in Intelligence saving throws. You gain expertise in history checks, and may remember information you have no rightful access to with ease. When you make a skill check, you may add your intelligence modifier to the roll. 

Charisma - You have disadvantage against saving throws versus the frightened condition, and you cannot gain immunity from that condition.  You gain proficiency in Charisma saving throws. When you look upon a creature, you can determine the things they're immune to, resistant to, and vulnerable to at a glance. If you attack a creature with no vulnerabilities, your attacks deal additional damage equal to your barbarian level and ignore resistance. 

Intimidating Presence

Beginning at 10th level, when you make intimidation checks, you can do so with your strength, intelligence, wisdom or charisma scores. When you would make an attack, you can instead make a special kind of verbal or magical grapple, using a contested intimidation check versus your intended target. If you succeed on the contest, the target suffer one of the following penalties, for the next minute ( a creature can only suffer from each penalty once):

Demoralized - Their attack rolls have a minus to hit equal to your proficiency bonus.

Outwitted - Choose a damage type, except bludgeoning, piercing and slashing. When the target deals damage of that type, it deals half the usual amount. 

Enchanted - The creature regards you in a friendly manner, obeying small, non dangerous requests, and they cannot use harmful abilities on you directly (area of effect disregarded). 

A creature who is affected by one of these penalties may use their action to make a contested intimidation or insight check against your intimidation check, ending the effect on a success. The effect also ends if the creature is completely unaware of your presence.  

Villainous Mastery

Starting at 14th level, your monstrous form is finally realized in it's entirety. With one last bargain, you take on another weakness in order to gain amazing power. 

Silvered Weapons - You are vulnerable against weapon attacks made with silvered weapons against you, and cannot be resistant or immune to that damage. You have advantage on perception checks, and attacks against you cannot have advantage. 

Holy Water - While in direct contact with holy water, you take 10d8 acid damage at the start of each of your turns, and your subclass abilities no longer function until it is removed. Removing holy water splashed on you requires two actions, or an appropriate effect, such as the spell Create and Destroy Water. When a creature takes an action other than the attack or multiattack action, you can use your reaction to first move up to your movement speed, then make two melee attacks against that creature if you can reach.  

Sunlight - If you are immersed in sunlight (clothing can block this) or magical light from a spell greater than 4th level, you take 20 radiant damage at the start of your turns, and cannot heal any hit points until not under this effect. You have a flying speed equal to your walking speed, and once per turn, your attacks with natural weapons or unarmed strikes drain 2d4 strength from your target, empowering your strength score by the same amount for 1d4 minutes. If you are missing hit points, this attack heals you for the amount it dealt. 

In addition, you maximize the power of all your accrued weaknesses. for each vulnerability you have, and/or each weakness you incur with this subclass, your maximum hit points increase by two max rolls of your hit dice plus two times your constitution modifier.