Way of the Sealed Shrine
You belong to a secret sect of monks who have mastered the art of sealing malevolent beings within the bodies of specially trained practitioners. These vessels, known as "Demon Bound," are chosen for their strength of will and purity of spirit. They are entrusted with the task of containing the great evil that threatens to destroy the world.
As a member of this sect, you have undergone rigorous training in both physical combat and spiritual discipline. Your ironclad resolve and mental fortitude allow you to withstand the corrupting influence of the malevolent entity sealed within you.
Through your spiritual training and your unshakable will, you are able to harness the dark energy of the sealed evil, using it to fuel your abilities and wreak righteous havoc on your enemies.
Unearthly Aspect
At 3rd level, you gain access to the unearthly language of Eldertongue. Eldertongue sounds like madding gibberish to any sane creature, but a creature under the effects of a madness-inducing spell (i.e. crown of madness, etc) or hex spell can understand it perfectly.
ART GENERATED USING MIDJOURNEY
Demon Bound
Also at 3rd level, you gain the ability to tap into the power of the malevolent entity sealed within you. As an action, you can spend 2 Ki Points to enter a state of heightened awareness, allowing you to borrow the power of your bound being to gain access to its knowledge and abilities for 1 minute.
While in this state, you gain temporary hit points equal to your monk level plus your Wisdom modifier and your first unarmed strike deals an additional 1d6 necrotic damage You also gain an additional power depending on the type of bound entity inside you that you choose from the list below when you gain this feature;
The Mage. You can spend 1 Ki Point to cast a cantrip from the Wizard spell list.
The Beast. You have advantage on perception and survival ability checks to track creatures.
The Spirit. You can move through other creatures spaces as if it where difficult terrain, creatures cannot make reactions against you if you move through them and you take 1d10 force damage if you end your turn within another creatures space.
The Outsider. Your unarmed strikes deal psychic damage and gain a range of 10ft. You also can cast mage hand at will.
However, using this ability comes with a cost. The malevolent entity sealed within you becomes more powerful and difficult to control, causing you to suffer disadvantage on Charisma checks. Additionally, if you are reduced to 0 hit points while in this state, the bound breaks free and takes control of your body, causing you to become an NPC under the DM's control until the demon is resealed or exorcised. Use this power wisely, as the consequences of losing control can be dire.
Monsters Physiology
At 6th level, your body has slowly become more adept at being the bound ones prison granting you a damage resistance depending on the type of bound being inside you;
The Mage. You gain resistance to fire damage.
The Beast. You gain resistance to acid damage.
The Spirit. You gain resistance to necrotic damage.
The Outsider. You gain resistance to psychic damage.
Additionally, when you hit a creature with an unarmed strike you can expend any number of temporary hit points you have to deal a additional necrotic damage to that creature equal to the number of temporary hit points expended.
Inheriting Darkness
At 11th level, the seal over the being within you begins to weaken, your resolve keeps the bound at bay but its powers seep further into you. You can choose 2 Eldritch Invocations from the warlock class that you meet the requirements for. You can treat your monk level as warlock levels and if the invocation calls for you to expend a spell slot you can use the feature one per long rest.
Demon King
At 17th level, your seal is now broken but you will never allow the bound to ever escape their prison. You can as an action, expend 11 Ki Points to steal the bounds form casting the shapechange spell.
When you cast this spell in this way your bound being determins the creature type you can choose from for your new form;
The Mage. Fiend
The Beast. Monstrosity
The Spirit. Undead
The Outsider. Aberration