Way of the Demon Fist

Whether through some dark arcane ritual or a deal with powers from the deepest pits of the Abyss, one empowered by the Demon Fist is the reciepient of a dark power by mingling their soul with one of a demon. Generally those who seek the power of the Abyss are those who were exiled from their monestaries for their thirst for power. Others are unfortunate victims of cruel experimentation by their Masters.

Most cursed with the aspect of a demon are shunned by monestaries, being seen as abominations. However exceptions exist, as those who can master the negative turmoil within them can rise as great masters of enlightenment. Thus, an adherent to the path of the Demon Fist is one wrought with the struggle of either combating their newfound demonic nature or embracing it to strike down their foes with overwhelming strength.

Demon Hide

At 3rd level, you can draw from the demonic power within you to harden your skin in response to damage, allowing you to absorb blows that would otherwise be fatal. Whenever you take damage that isn't Radiant or Psychic damage you can, as a reaction, spend additional Ki points to reduce the damage taken by 1d8 + your Constitution Modifier + half your Monk Level (rounded up). Each additional Ki point you spend adds another 1d8 to the roll.

At 17th level, you are able to extend this protection temporarily to cover your entire body, transforming you into a being closer to demon than man. Anytime you use this ability, you gain resistance to all damage except Radiant and Psychic until the end of your next turn.

Abyssal Anatomy

At 3rd level your body is forever changed by the dark rituals, allowing you to sniff out traces of fiendish essence around you. You have advantage on Survival checks to track fiends, and you have advantage on Perception checks to percieve fiends in disguise.

Additionally, you can understand and read Abyssal, although you cannot speak it. You also gain a permanent disfigurement relating to the demon that inhabits you. (i.e. horns, a tail, a clawed hand, etc.)

Hellfire Strike

At 6th level, you learn how to channel damage into power, unleashing a blast of your corrupted Ki in response to attacks. Whenever you reduce damage to 0 using your Demon Hide ability, the next unarmed strike you make on a hit deals an additional 1d8 fire or necrotic damage (your choice) per Ki point spent on the ability. You cannot spend more Ki points once you have reduced the damage to zero. If you do not make an attack on your next turn the energy is wasted.

At 17th level, you are able to use incoming damage as energy to summon the flames from the darkest pits of the Abyss. Whenever you reduce incoming damage to 0 with Demon Hide, you can as an action during your turn unleash a wave of Hellfire. Dealing 2d8 fire or necrotic damage per Ki point spent to all creatures within a 20ft radius centered on you unless they succeed in a Dexterity saving throw against your Monk DC, otherwise they take half damage. This energy is lost if you do not use the ability at the end of your next turn.

Abyss Walker

At 11th level the demonic essense within your soul allows you to traverse great distances by stepping through the treacharous dark of the Abyss. As an action, you can spend 4 Ki points to cast the Dimension Door spell without material components.

At 17th level, anytime you use this feature, you emerge from the Abyss shrouded in its corruption. Any creature within 10ft of you must make a Constitution saving throw against your Monk DC or be poisoned. They can repeat this saving throw at the end of their turns, but only if they leave its area of effect. The aura dissapates after 1d6 rounds.

Credit: Hobgoblin Cleric of Hadregash - Ironfang Invasion - Pathfinder