At one point in every man's life, he wishes to be the strongest in the world. Eventually, most men give up on this dream, but not you. You have discovered a dark power that resides deep within martial artists. Most are completely unaware of its existence, due to the want and love for extreme violence being the catalyst that draws it out. This power would be considered by many to be evil, but, should your warrior spirit and sheer force of will be strong enough, you can harness this power for however you see fit.

 

this is a port of the Way of Ogre Subclass from the Page on DND Wiki's Homebrew section

Demon Back

At 3rd level, an ancient and dark power awakens within you in times of stress. When you drop below half your maximum Hit Point, you enter a state of unmatched power. While in this altered form, the muscles in your back shape into the form of a smiling demon and you gain the following benefits:

• You gain temporary Hit Points equal to your Monk level + your Wisdom modifier.
• After the first instance of taking a type of damage, you gain resistance to that damage type.
• Your unarmed strikes deal an extra 1d4 bludgeoning damage. This damage increases when you reach certain levels in this class: to 1d6 at 6th level and to 1d8 at 14th level.

Your Demon Back and all its benefits will dissipate after 1 minute. It can also end early if you are knocked unconscious. You can also end it on your turn as a bonus action.

Once your Demon Back awakens, you must finish a long rest before it will show itself again naturally. However, you can use a bonus action and spend 1 ki point to reawaken it forcefully, if you are still under half your maximum Hit Points.

Ferocious Warrior Spirit

Starting at 3rd level, an unsettling aura surrounds you at all times. You gain proficiency in Intimidation, or expertise if you are already proficient. Also, any creature that sees you awaken your Demon Back must succeed a Wisdom saving throw or become frightened of you for 1 minute. A creature that fails the save may repeat it at the end of each of its turns, ending the effect on a success. A creature that succeeds the save is immune to this effect for the next 24hrs.

Deadliest Fist

Starting at 6th level, your fists become even deadlier weapons. When you roll a 19 on an attack roll with your unarmed strikes, you can spend 1 ki point to make it a critical.

This extends to rolls of 18 or 19 at level 17.

Ultimate Violence

At 6th level, your knowledge of biology allows you to obliterate your opponent's body with your bare hands. When you land a critical hit with your unarmed strikes, you can spend 2 ki points and choose 1 of the following effects to inflicts on the target until the end of your next turn:

• Break the Leg. The target is knocked prone and their speed is reduced to 0.
• Concuss. The target is blinded and deafened.
• Shatter the Hand. The target has disadvantage on attack rolls and cannot perform the somatic components of spells.

The Power of Imagination

Starting at 11th level, you master the art of mimicry and can use your body as anything crafted for combat. As an action, you can choose a single creature within 30ft and enter a special stance that mimics their very being. While in this stance, your unarmed strikes assume the damage dice of one of their melee attacks. When you do this, the attack must deal bludgeoning, piercing, or slashing damage, and you ignore any additional elemental damage or effects the attack would do. If you wish you copy a different melee attack, you must use another action and spend 1 ki point to change stances.

You can only mimic creatures whose CR is equal to or less than your level, and the stance only lasts for 1 minute.

The Pinnacle of Physical Force

Starting at 17th level, you master the destructive power of martial arts and become a true demon, an ogre. You can now awaken your demon back at any time as a bonus action. The first time you do so is free, but still costs 1 ki point for subsequent awakenings. Additionally, while your demon back is awoken, you can add both your Strength and Dexterity modifiers to the hit and damage rolls of your unarmed strikes.