Way of Dominion

A monk who follows the Way of Domination believe their body to be a holy vessel to contain and control the domineering power of a Fiend. Binding the remnants of a Fiend to their spirits, their ki radiates in thick billowing clouds from their bodies. They see the dark ki emanating from their person as a means of destroying evil with evil and as a representation of dominion. The powerful changes to their inner self and body can lead one to become a mighty force for good - fighting for what is right and just - or a terrible force for evil - fighting at the will of the Fiend they bond themselves to. 

Fiendish Vigor

Starting when you choose this tradition at 3rd level, you can manipulate your ki to boost your physical capabilities. After finishing a Long Rest, you can choose from one of the following abilities:

Fiendish Vigor: Vitalis. Gain temporary hit points equal to 1d10 + your Wisdom modifier. Make this roll after finishing a Long Rest. Any lost temporary hit points are refreshed at the end of a Short Rest.

Fiendish Vigor: Potestas. Gain a temporary boost to your unarmed attacks - changing the Unarmed Attack die to a 1d6 + your Wisdom modifier. At 5th level, the attack die increases to 1d8, at 11th level to 1d10, and at 17th level to 1d12.

Fiendish Vigor: Ferrum Corpus. Gain temporary resistance to non-magical slashing, bludgeoning, and piercing damage.

Infernal Form

At 6th level, you can utilize your ki to summon a protective shell around you in the appearance of the Fiend that resides within you, and bolster your offensive capabilities. As a bonus action, you can spend 2 ki points to unleash your Infernal Form. While in this form, you incur the following effects on yourself:

• The shell like aura placed around your body wreathes you in an otherworldly flame, granting you resistance to Fire and Cold damage. If an enemy attacks you with a melee attack they are dealt 5 points of Fire damage upon contact.
• The spectral form of your inner Fiend unleashes from within you a terrifying might. During this time, your unarmed attacks gain an additional 1d6 + your Wisdom modifier of Fire and Bludgeoning damage.
• The immense pressure from your ki sends shockwaves of shivers down the spines of those within sight of it. When you unleash your Infernal Form, all creatures within 60 feet of you must pass a DC 10 + your Wisdom modifier or be Frightened of you for 1 minute. 

You must finish a long rest before you can use this feature again. You revert back to your normal form if you are incapacitated or dying.

Nightmare

Beginning at 11th level, you can enter a special meditation that surrounds you with a nightmarish aura. At the end of a long rest, you gain the effect of a Fear spell that lasts until the start of your next long rest (the spell can end early as normal). The saving throw DC for the spell equals 8 + your Wisdom modifier + your proficiency bonus.

Dominion

At 17th level, you gain the ability to Banish one creature to the Lower Planes with the aid of the Fiend within you. The creature must make a Wisdom saving throw of DC 10 + your Wisdom modifier + your proficiency bonus. If it fails, the creature is banished to the Fiendish Lower Planes and will remain there eternally until you end the action.

You can have only one creature under the effect of this feature at a time. You can choose to end the banishment without using an action.