Fiend Vessel

Fiend Vessel's use the power they either received or found to hunt Fiends and other monsters. The more the Fiend Vessel uses this power will cause their form to change into a more noticeable fiendish look. Such as patches of skin turning into red scales, eyes changing color, or even horns starting to grow from their head. Eventually turning their form itself into a fiend giving them control of those weaker than them. 

Curse of the Fiend Vessel

At 3rd level, you have learned how to harness the power of fiends against them and other creatures. As a result, features common in fiends such as horns or scaly skin start to develop on your body.

Hunters Mark now deals fire damage and ignores resistances and immunities a creature would have against fire.

Unarmed Strikes now do 1d6 (Slashing damage) + DEX mod

Binding of Souls

At 7th level, you have learned to bind your targets soul to your own. Doing so allows you to to ignore the targeted creatures resistances and immunities the target has. If the target tries to target you with a spell or attack they must succeed a Wisdom save equal to your spell save DC. 

Your fiendish features start to spread and seem to be more prevalent throughout your body

Fiendish Form

At 11th level, the curse of the fiend has fully enveloped your form making you sprout wings from your back. You gain a flying speed equal to your walking speed and have resistance to fire and poison damage.

You also gain 120ft of darkvision

Fiendish Rule

At 15th level, due to your fiendish form you have developed power over fiend's that are weaker than you and have the power to summon them to your aid. Summon 1d4 fiends of CR 5 or lower (Dm's choice of fiends)

Your presence now instates fear (Frightened) in any opposing creature within 35ft of you unless they succeed a DC 16 Wisdom Save.