Ki is the magical energy of all living bodies, and like souls, a trace of one's Ki lingers on after death. This residue is most notably found in walking corpses where the spirit has been made to walk the earth again. Chained back to the land of the living, keeping their souls from reaching afterlife. The Way of the Fiend monk manipulates this residual energy. This art is unorthodox to other monasteries who don't when it first appeared.

Draft Of The Blood War

The unending Blood War calls for your presence.

At 3rd level you become resistant to fire and poison damage. You must also choose which side of the war to join; Asmodeus or Demogorgon.

Asmodeus' Terrifier

You can speak, read and write in Infernal. You have Advantage on Wisdom saving throws against being Frightened.

You can spend 1 Ki point and force all creatures within 20ft of you to make a Wisdom saving throw (DC = 10 + CHA), you cannot use this trait if you are: frightened, grappled, incapacitated, paralyzed, petrified, prone, restrained or stunned. Friendly creatures roll with advantage. On a failed save, the creature is frightened and can reroll this save on every one of its turns. On a success, the creatures are immune to this trait for the next 24 hours.

At 6th level the range increases to 30ft and you can force a number of creatures equal to your Charisma modifier to roll with disadvantage against this trait's primary effect. And you can cast the Fear and Find Steed spells, both at 2nd level once per long rest each.

Demogorgon's Firestarter

You can speak, read and write in Abyssal.

You can spend one Ki point to set yourself ablaze for 1 minute. For the duration each creature that either hits you with a melee attack or gets hit by one of your melee attacks takes 1d4 fire damage, and flammable objects you touch that aren't being worn or carried, are set on fire. You can spend a second Ki point to concentrate the flames on the palms of your hands, immediately ending the primary effect and casting the Burning Hands spell at 1st level.

At 6th level your flames now deal 2d4 fire damage, and you can choose to cast the Flame Blade spell at 2nd level instead.

Fiendish Enforcer

At 6th level your efforts in the Blood War have yielded fruitful results. You gain new boons depending on who you sided with.

Eyes Of The Abyss

Demogorgon's blessing has granted you a smidge of his power.

Once during your turn, you can choose one creature you can see within 60ft of you. The target must make a Wisdom saving throw (DC = 10 + CHA), on a success nothing happens and the target rolls with advantage the next time it'd roll this save. (A Blinded creature automatically succeeds this Saving throw)

On a failure, roll 1d6 to determine which effect is activated. 

 

(1-2): Beguiling Gaze: The target is Stunned until the start of your next turn.

(3-4): Confusing Gaze: The target suffers the effect of the Confusion spell without making a saving throw. The effect lasts until the start of your next turn.

(5-6): Hypnotic Gaze: The target is Charmed by you until the start of your next turn. You can choose how the charmed target uses its action and movement but can't force the target to attack.

 

While any of the effects above is active, you lose 1 AC to attacks from all creatures that aren't your target and you can't take reactions. The effects end early if you no longer have a line of sight on the target.

Faustian Conviction

You become immune to the Frightened condition, gain advantage against being charmed or ending the Charmed condition and you can now make a Faustian Deal with mortals.

You can make deals with any creature with an Intelligence score of 8 or higher. To prepare a Faustian Deal, you'll need: an ink bottle (ounce), an ink pen, 1d4 sheets of paper, a signet ring and sealing wax. All materials, except for the ink pen and signet ring are consumed in the process. 

The creature must roll a Wisdom saving throw (DC = 10 + your Charisma modifier) on a success the creature does not accept your deal and all materials spent go to waste, on a failure the creature accepts it and both of you must sign the contract with blood, both parties take 1 damage.

The specifics of the deal should be worked out with your DM and you must complete your part of the deal to get the creature's soul. However, a Faustian Deals always cost the creature their soul, reincarnating into a Lemure in Avernus once they die.

You can accumulate up to 5 souls, you can spend souls to receive a magical item of your DM's choosing. The rarity of the item will depend on how many souls you spend on it:

Soul Cost	Rarity
1	Uncommon
2	Rare
3	Very Rare
4	Legendary
5	Artifact

Soul collector

At 11th level your master gives you tools to grow the strength of his army.

You can cast the Banishment spell once per short or long rest but must always send the target to Avernus if you serve Asmodeus and to Pazunia if you serve Demogorgon.

Fiendish Hierarchy

At 17th level your fiendish sovereign has granted you a significant title within their ranks. You learn the Plane  Shift spell, that can be cast once per week. And a second spell depending of who you serve.

Margrave of Nessus

The Supreme Master of Baator has granted you an estate close to his seat of power in Nessus, the ninth layer. Within its landscape is a featureless plain with jagged edges from north to south, with the River Styx flowing through a small offshoot, floating in an endless red void.

Your estate consists of a 1 square mile plot of barren land. When you gain access to this feature you can choose the general look of the building on your land. However, it always has 5 rooms that you can decorate as you wish.

You can use the Plane Shift spell granted by this feature only to travel in and out your estate. When traveling out of it you are sent to wherever you were right before you entered the estate.

You also learn the Infernal Calling spell and can cast it once a day. You must always summon the same devil and you are granted that devil's talisman.

Marquess of The Gaping Maw

The Prince Of Demons has granted you and estate on the 88th layer of the Abyss, the Gaping Maw. Within its landscape is a great sea of briny water broken by tall, sharp, ugly, rocky promontories.

Your estate consists of a 1 square mile plot of barren land. When you gain access to this feature you can choose the general look of the building on your land. However, it always has 5 rooms that you can decorate as you wish.

You can use the Plane Shift spell granted by this feature only to travel in and out your estate. When traveling out of it you are sent to wherever you were right before you entered the estate.

You also learn the Summon Greater Demon spell and can cast it once a day. You must always summon the same demon, which you know the true name of.