Beast Berserker (Cerberus)

A barbarian subclass with a Cerberus theme that gets stronger the more hit points it losses.

Beast Rage

-Whenever you enter rage your body shifts into a frightening beast-like appearance. Additionally when raging you gain one of the following natural weapons: (You may choose a different natural weapon per instance of rage.)

- Beast Bite: Your beast form’s mouth grows more prominent and vicious. Bite does 1d6 damage and uses strength or Finesse, once per turn when dealing damage with the bite you regain a number of hit points equal to your proficiency bonus + your constitution modifier.

- Beast Claws: Your beast form’s hands grow dangerous claws. Claws do 1d8 damage and use Strength or Finesse. Once on each of your turns when you attack with a claw using the Attack action, you can make one additional claw attack as part of the same action.

Berserk

For each time you lose a number of hit points equal to your constitution modifier you gain +1 to AC and +1 to attack and damage rolls. These bonuses persist only for the duration of your rage, and you lose all bonuses from this feature when your rage ends. After your rage ends, gain a point of exhaustion for each time this feature activates after the fifth time.

Additionally, your natural weapons gained during rage count as magical for the purpose of overcoming resistances. 

Cerberus Soul

-This feature is applied after failing the save from the 11th level feature Relentless Rage, and can only activate while you have no points of exhaustion.

-If you are raging when you are reduced to 0 hit points, instead of falling uncousing and ending your rage your rage persists and you regain half your hit points. 5 minutes after this feature activates your rage ends and you gain 3 points of exhaustion, your rage ends even if you have the 15th level feature Persistent Rage.

-If you are once again reduced to 0 hit points after this feature has already activated, instead of falling uncousing and ending your rage your rage persists and you regain half your hit points 5 minutes after this feature activates your rage ends and you gain 6 points of exhaustion, your rage ends even if you have the 15th level feature Persistent Rage.