Monastic Tradition - Way of the Deceiver
While many monks use their training to access inner peace and enhance their connection to the world around them, so do it to master more profane powers. Those who follow the Way of the Deceiver seek to master hell's gifts, whether to use against them, or for their own benefit.
Those who follow the path of the Deceiver are masters of deception, turning allies against one another and draining the life force from their targets. While those who follow the Way of Shadow may be masters of infiltration in darkness, those who follow the Way of the Deceiver are masters of infiltrating by walking in the front door with a smile on their face. Experts who follow this path may find themselves functionally immortal, exchanging the souls of others to stave off the abyss that calls to them.
Abyssal Ki
Your monastic tradition focus, dedicated to the ways of devilish infiltrators, reforms your body to become more magical. Your Unarmored Defense feature and ki-save DC can now use Charisma, instead of Wisdom if you so choose. In addition you gain proficiency with the Deception skill.
When you reach 10th level in this class, you can apply your proficiency bonus twice to any Deception ability checks you make.
Energy Drain
Your hellish energies can drain the lifeforce of those you touch. On a successful unarmed strike, you deal additional psychic damage equal to your charisma modifier. In addition, if you're successfully grappling a creature, they take the same amount of damage at the end of their turn. The extra damage dealt by energy drain reduces the target's hit point maximum by the same amount.
Energy Drain's damage from an attack can only be applied once per turn. From 6th level, if you have 0 ki remaining, you regain 1 ki when you deal damage with Energy Drain. From 11th level, it can apply twice each turn. From 16th level if you are below half hit points when you deal damage with Energy Drain, you regain hitpoints equal to the psychic damage dealt.
Shapechanger
From 6th level as an action you can polymorph into a small or medium humanoid of your choice, or back into your true form. Other than your size, your statistics are the same in each form. Any equipment you are wearing or carrying isn't transformed. If you die, you automatically revert to your true form.
You can use this feature a number of times equal to your Charisma modifier, refreshing on a long rest. While you have no uses available, you can spend 1 ki point to use this feature again.
Fiendish Charm
From 6th level as an action choose one humanoid you can see and can see you within 30 feet. It must succeed on a Wisdom saving throw or be magically charmed for 1 minutes. The charmed target obeys your verbal or telepathic commands. If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, and the target is immune to your Charm for the next 24 hours. You can have only one target charmed at a time. If you charm another, the effect on the previous target ends.
You may spend additional ki points to extend the charm. For each additional ki point spent, the duration extends to 10 minutes, then 1 hour, then 8 hours, then 24 hours for 4 ki points spent. You can spend these ki points at any time while the Fiendish Charm is active, they do not have to be part of the initial action, but the Charm's duration cannot be extended beyond 24 hours.
You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.
While you have no uses available, you can spend 2 ki points to use this feature again.
Telepathy
At 11th level you can speak telepathically to any creature you can see within 60 feet of you. Your telepathic utterances are in a language you know, and the creature understands you only if it knows that language. The creature can respond telepathically if they're friendly or are Charmed by you.
You may also cast the Detect Thoughts spell without providing any components. You can do so once, before requiring a long rest to use again. While you have no uses available, you can spend 2 ki point to use this feature again.
Devilish Resistance
From 11th level you gain resistance to lightning damage.
Infernal Contract
By reaching 17th level you are now able to create your own Infernal Contracts, using both your own powers and those of the abyssal realms to benefit others - in exchange for their souls. By convincing a creature to sign a contract with you, they gain the effect of one of the Charms listed below, or the effect of one spell up to 7th level, and in doing so agree that their soul now belongs to you. An Infernal Contract cannot be made with someone under magical or mundane compulsion, such as your Fiendish Charm, alcohol, or any spells that might impact their decision-making. Those who make the decision must do so with sound mind, though if they happen to be in distress or danger, that's acceptable.
Creating and signing a contract takes 10 minutes of uninterrupted negotiation and review between the two parties.
Whenever you would die, you can instead expend a soul in your possession and have them die in your place, negating the damage or effect that would kill you. If you would die from a failed death saving throw, you instantly regain hit points equal to the expended soul's current hit points. If you expend a soul in such a way, the creature that died in your place is sent to Hell immediately and cannot be restored to life, outside of Divine Intervention or a Wish spell.
There is no limit to the number of souls you may have in your possession, however if you die without any souls to expend in your place after making at least 1 successful contract, you are likewise sent to hell without any chance of resurrection - though the more souls and contracts you accrued during life may advance you high in the devil's hierarchy.
In addition, so long as you have at least one additional soul in your possession, you gain an additional +1 to all attack rolls, saving throws and ability checks, and your Martial Arts die is increased to 2d6, and you cannot die of old age.
Charms
Agelessness - You no longer age through normal means and cannot die of old age. You may also choose to reverse your aging back to whatever age you choose, but only when the contract is initially signed. Magical aging effects still take effect as normal, however.
Diabolical Inspiration - When you make an ability check, an attack roll, or a saving throw, you can use this charm to gain advantage on the roll. You can do so 9 times, before the charm's effects are expended.
Charm of Conscription - As an action you can summon a horned devil, or a barbed devil, or two bearded devils, or three spined devils, which appear a square within 30' of you. The devils obeys your commands and serves (with some protest) for 9 days or until it dies, at which point the charm's effects are expended.
Hellish Rebuke - A hellish halo appears over your head. You may cast the Hellish Rebuke spell at 9th level as a reaction up to 9 times. Once used, the charm's effects are expended and the halo vanishes.
Many Tongues - You can speak and understand all languages, you gain proficiency in the Deception and Persuasion sfillls, and you can add double your normal proficiency bonus when using those skills.
Adamant - This charm has 9 charges. You can use an action to expend 1 charge of this charm to gain 30 temporary hit points. For one hour after doing so, you also gain immunity to the Charmed and Frightened conditions.
Animal Conjuring - As an action you may cast the Conjure Animals spell at 3rd level. You can do so 9 times before the magic of this charm is expended.
Darkvision - You gain Darkvision out to 120'. If you already had Darkvision from another source, it instead increases the range by 120'.
Feather Falling - You gain the permanent effects of the Feather Falling spell, which is not expended after a fall, nor can it be dispelled by Dispel Magic.
Restoration - This charm has 9 charges. You can use your action toexpend 1 charge to cast Lesser Restoration, or 3 charges to cast Greater Restoration.
Slayer - One weapon of your choice becomes a Slayer weapon, dealing an additional 3d6 damage on each successful attack against a creature type of your choice. This lasts for 9 days before the magic is expended.
Vitality - As an action you can imbue yourself with vitality. You are instantly cured of all exhaustion, disease, poisons, and your hit points are fully restored. You can use this charm 9 times before the magic is expended.