Much like the demons, Way of the Oni monks are the stuff of nightmares. Paired up with an ancient evil spirit, they walk the knife’s edge of self-control or absolute carnage.

Oni's Frenzy

At 3rd level, you gain the ability to enter a demonic frenzy, harnessing the Oni’s rage to empower you. As a bonus action, you can spend 1 Ki point go into an frenzy. 

While frenzied, you gain the following benefits:

• You have advantage on Strength checks and saving throws.
• When you make a melee weapon attack or unarmed strike using Strength, you gain a bonus to the damage roll equal to your Martial Arts damage die.
• You count as one size larger when determining your carrying capacity and the weight you can push, drag, or lift.
• Your Martial Arts damage die increases to its next stage during your frenzy. The increase change continues as follows: D4 -> D6 -> D8 -> D10 -> D12
• You have advantage on saving throws against being frightened or possessed.

Your frenzy lasts for 1 minute. It ends early if you are knocked unconscious or if your turn ends and you haven’t attacked a hostile creature since your last turn or taken damage since then. You can also end your frenzy with a bonus action.

Once you have frenzied the number of times equal to your proficiency bonus, you must finish a long rest before you can frenzy again.

While frenzied, if you kill a creature, end your frenzy early or the frenzy wears off, you must make a Wisdom saving throw to prevent the Oni from taking control. The DC equals 8 + your proficiency bonus + your wisdom modifier. At the end of each of your turns, you can make another Wisdom saving throw. On a success, you keep control of yourself.

Additionally, while frenzied, all damage is considered lethal and you may not incapacitate an enemy when you reduce their HP to 0.

The Power Within

Monks cursed with the Mark of the Oni can tap into the mystical energy of Ki that isn’t calm, but ferocious. At 3rd level, instead of spending time during a short rest meditating, you perform ancient rites with your body. This includes special breathing techniques that produce mystical energy that circles your body as if you were in the heat of battle.

As your body and soul adapts to the monstrous energy of the Oni, you gain the following benefits:

• You can use your Strength modifier instead of your Dexterity modifier for the purposes of determining your Unarmored Defense AC.
• You can use your Strength modifier instead of your Dexterity modifier when using the Deflect Missiles feature.

Oni's Strength

At 6th level, you learn to harness the Oni’s physical strength, hardening your arms in response to an incoming attack. Whenever you are hit by a melee attack, you can use your reaction to reduce the incoming blow. When you do so, the damage you take from the attack is reduced by 1d10 + your Strength modifier + your monk level.

Ki Consumption

At 11th level, you gain the ability to consume a creature's Ki. You must declare this action before making an attack and must be an unarmed strike. The creature must make a Constitution saving throw. If it fails, you replenish 2 Ki points. On a success, you regain 1 Ki point. If you deal the killing blow with this technique, you don't spend a use of this feature. You can only use this feature once per turn. You cannot gain more Ki points than your maximum Ki pool. This feature can be used as many times equal to your Wisdom modifier. You regain all expended uses after a short or long rest.

Additionally, you have mastered the possession and learned to sustain the curse. You now have advantage when making your save to maintain control.

True Potential

At 17th level, you learned to combine your body and soul with the Oni, harnessing all of its power. Anytime during your frenzy or when you enter a frenzy, you can spend 3 Ki points to fuse with the Oni. You gain the following benefits:

• You gain temporary hp equal to your Strength modifier + your monk level.
• You gain the effects of the Haste spell until the frenzy ends.
• You gain an additional attack during Flurry of Blows.

Your outward appearance resembles a split between you and your Oni.

This effect is suppressed while the Oni has control over you.