Your study of the hells and fire have inspired your fighting. The underworld and infernal influence have become one with your Ki.

Gifts from Below

Starting when you choose this tradition at 3rd level, you can use your devilish ki to duplicate the effects of certain spells. As an action, you can spend 2 ki points to cast darkness, burning hands, command, or hold person, without providing material components. Additionally, you gain the fire bolt cantrip if you don’t already know it.

Fiendish Shadows

At 6th level, you gain the ability to fade into the darkness. When you are in dim light or darkness, as a bonus action you can become invisible until the start of your next turn. If you make an attack against a creature, enter an area of bright light, or cast another spell, your invisibility drops.

Flaming Fists

By 11th level, your hellish powers cause flames to erupt from your fists. When you hit a creature with your Flurry of Blows feature, you can add 1d8 fire damage to each of your hits. You can do this a number of times equal to your proficiency bonus. You can declare this feature after knowing the result of your roll. Additionally, these flames ignite anything that isn't being carried or worn.

Brain Burn

At 17th level, you can violently ignite the mind of a creature in support of your allies . Whenever a creature within 5 feet of you is hit by an attack made by a creature other than you, you can use your reaction to  force the creature make a Wisdom saving throw equal to your Ki save. On a failed save the creature takes 8d10 fire damage or half as much damage on a successful save as flames erupt inside of their mind. You can use this feature once per long rest.