(This is an update from the original work, known as "Way of the Red Oni", in order to fix some issues I felt it had including spelling errors, grammatical errors, and issues with the character sheet. I did not create the original version, I just felt like it could use some tweaking. The original author is dreadedlord).

 

Aura of Destruction

When you first obtain this ability, your martial arts die grow in size in relation to the Monk Table in the Player's Handbook. For example: d4=d6, at level five a d6=d8, so on and so forth! At level 20, you can choose for your d12 martial arts die to become 2d6.

Upon falling down this dark, forbidden style, the monk has tap within his inner demon and unleash it upon the world. Using your bonus action, the monk body release a dark aura around one feet around him, his eyes glowing red and has this intent sense of rage (similar to Barbarian Rage). While in this form, you gain the following benefits if you aren't wearing heavy armor:

• -You have advantage on Dexterity Checks and Dexterity saving throws.
• -You gain an extra ten feet of speed, while as an bonus action can do a high jump up to 10 feet + Dexterity modifier ( EX: 1 means one feet).
• -You gain an extra +2 AC while you are in this form.
• -You have Resistance to bludgeoning, piercing, and slashing damage that are non-magical.
• -You gain a damage bonus equal to your Dexterity modifier on unarmed strikes and monk weapon attacks.
• -Your attacks are consider magical.

Your aura lasts for 1 minute. It ends early if you are knocked Unconscious, or if your turn end and you haven't attacked a hostile creature since your last turn or taken damage since then. You could only used this method up to three times, and then you must finish a long rest afterward. At 9th level, you now get four. At 13th, you now have five.

Eruption of blows

Flurry of blows gets an upgrade! Using up one Ki points as an bonus action, what normally would be only a additional two strikes, the Eruption of Blows now grants an additional three strikes with an additional 1d6 necrotic damage for the last attack. At 11th level,  the technique has been improved and now you can do an additional four strikes instead of three, with an added 2d6 necrotic damage (1d6 for the last two strikes). At 17th level, all four of the attacks of the Eruption of Blows has the added 1d6 necrotic damage.

Devil's Bolt

At 11th level, you have mastered the ability to manipulate the dark ki within your body and bring it out of you to form a ball of darkness within your hand. As an action, you may make a 60ft ranged attack (this is considered a monk weapon attack) by spending any number of ki points and firing the ball of darkness at your enemy, on hit you deal Xd8+ your dexterity modifier as Necrotic damage, where X is the number of ki points spent (example:1d8, 2d8, 3d8, etc.).

Kuruoshiki Oni

 

At Level 17, after months and years of mastering this form, you can use four ki points as an bonus action to now enter the final, powerful state known as the "Mad Ogre". Your skins turn red, followed by your hair turning pure white, then two pair of black horn burst out from you forehead. You can feel intense power exploding within you. You also become the size of the Red Oni, growing an extra size, becoming Large-sized for a limited time. While in this form, you gain the following benefits if you aren't wearing armor:

• -You have advantage on Dexterity Checks and Dexterity saving throws, while your enemies have disadvantage on any close-range attacks within five feet of your dark aura.
• -You gain an extra +4 AC while you are in this form.
• -Instead of getting an extra walking speed, this form grants you the ability to fly up to 30ft, like a Oni.
• -You become immune to bludgeoning, piercing, and slashing damage that are non-magical.
• -When you attack, your arms and feet become coated in bright, almost superheated orange glow. You gain a damage bonus equal to twice your Dexterity modifier + your proficiency bonus on unarmed strikes.
• -As an action, you can use your special attack, the "Erupting Demon", on a single target. It is a move to which you strike at your foe with all your strength, your arm coated in that bright orange glow, a powerful surge of demonic energy to going through the enemy, causing a powerful explosion to erupt right behind the target. The attack is a single unarmed strike dealing 5d8 bludgeoning damage, 5d8 neurotic damage, and your dexterity modifier(totaling 10d8 + dex mod). You can use this move once per long rest.
• -Your attacks are consider magical.

Your Oni Form lasts for 1 minute. It ends early if you are knocked unconscious, if your turn ends and you haven't attacked a hostile creature since your last turn or taken damage since then. You could only used this once per long rest. Activating this ends aura of destruction.