Ebb and Flow

As the life from your enemies comes and goes you always remain standing from what could be assumed demonic influence. The understanding of the body, magic, and natural toughness made you a force to be reckoned with.

You gain the following:

Essence score. You gain an essence score which ranges from 0-100.  When your essence score reaches 100 you gain a hit dice. 

Battle Favor.
When you crit with a melee weapon make a constitution save. Record the result and add it to your essence score. 

Hardy.
When below 50% of your max health (rounded down) every hit causes your blood to boil and a refresh, a natural resistance within your body. Make a constitution save to record the result and add it to your essence score. 

Disadvantage on Charisma Checks
Advantage on Death Saving throws