This is a personally revised version of the Living Nightmare in Steinhardt's Guide to the Eldritch Hunt.

Otherworldly Transformation

The powers within your eldritch flesh begin to awaken. Due to your terrifying appearance, you can use your Constitution instead when you make Charisma (Intimidation) checks, and you have advantage on Intimidation checks.

Your flesh hardens and your reflexes grow, empowered by otherworldly ichor. While you are not wearing any armor, your Armor Class equals 11 + your Dexterity modifier + your Constitution modifier. You can wield a shield and still gain this benefit.

Armory of Mutation

As a bonus action or as part of making an attack roll, you can mutate your body, transforming any number of your limbs into various weapons of destruction or back to normal. They count as one-handed simple weapons for you, and you add your Strength or Dexterity modifier (your choice) to the attack and damage rolls when you attack with them, as normal. When you use these weapons, you drop any items you are holding with the limbs involved. Some of the Living Nightmare features require your target to make a saving throw; the DC is calculated as follows:

Living Nightmare save DC = 8 + your proficiency bonus + your Constitution modifier

You choose the weapons' form each time you make an attack roll. When you do so, you can use an additional mastery property of your choice (choosing from all available properties).

Needler. One of your limbs grows into a razor-sharp blade. It counts as a rapier with the Finesse and Light properties, and deals 1d12 piercing damage on a hit. When you attack with the Needler at least once when taking the Attack action, you can use a bonus action to make a single additional attack with it.

Arm Club. One of your arms turn into a black-tinted bone club. It counts as a glaive with the Heavy, Reach, Versatile (2d10) and Push (mastery) properties, and deals 2d8 bludgeoning damage on a hit.

Lashing Tendon. One of your limbs replaces its bones with tough coiled ligaments, allowing it to stretch a disturbing distance without breaking. It counts as a lance with the Reach (15 feet) and Topple (mastery) properties, and deals 1d12 slashing damage on a hit.

Sacrificial Bow. One of your arms turn into a bow, firing ammo made of your flesh. It counts as a crossbow with the Light and Range (150/600) properties, and can be fired with the transformed limb. It deals 1d12 piercing damage on a hit. When you hit with this weapon, you may take necrotic damage up to your Fighter level to add twice that amount to the attack's damage.

Alternatively, you can transform your limb into a shield:

Sinister Aegis. As a bonus action, you can temporarily turn one of your arms into a fleshy shield. It counts as a shield that grants a +2 bonus to your AC. Whenever you are targeted by an attack that you can see, you can use your reaction to summon the shield (if you haven't have one) and fortify it, gaining an extra +3 bonus to AC until the start of your next turn.

In addition, your eldritch flesh is capable of devouring some magical items to gain their properties. As part of a short or long rest, you can devour the following types of magic items, destroying the item and absorbing their properties:

• A melee weapon with the Finesse and Light properties; its properties are added to your Needler.
• A melee weapon with the Heavy property; its properties are added to your Arm Club.
• A melee weapon with the Reach property; its properties are added to your Lashing Tendon.
• A ranged weapon; its properties are added to your Sacrificial Bow.
• A shield; its properties are added to your Sinister Aegis.

If you absorb another magic item of the same type, the previous properties added from that type is lost. If a magic item requires attunement, you can only manifest the additional properties of the item if you are attuned to it. You are not considered as holding the magic item or having it on your person if none of your limbs is transformed to that item. You can attune or unattune to any number of devoured items with a ritual that takes 10 minutes, which can also be done as part of a short or long rest.

Devourer of Death

You can use the voracious hunger that dwells within your accursed body to devour the corpses of your foes and mend your flesh. When you stand within 5 feet of the corpse of a creature that has died less than 1 week ago, you can use your bonus action to touch it and let your eldritch appendages devour it. You empower yourself in one of the following ways of your choice:

• You regain a number of hit points equal to five times the creature’s CR (rounded up). Any excessive healing is converted to temporary hit points.
• You gain a bonus to the next ability check, attack roll or saving throw you make; the bonus equals the creature's CR (rounded up), up to a bonus of +10. This does not stack.
• You gain an additional Action. This can only be chosen if the creature's CR exceeds half of your level (rounded up).
• Until the end of your next turn, all your melee attacks with Eldritch Weaponry has their range increased by a number of feet equal to five times the creature’s CR (rounded up).

In addition, for the next 24 hours, you can use an action to assume the creature’s appearance and voice, though none of your game statistics change. You stay in the new form until you use an action to revert to your true form or until you die. While in the new form, you may use your Constitution instead for Charisma checks to disguise as the creature.

After 24 hours, you lose the ability to take the appearance of the consumed being.

Ascension of Flesh

Your eldritch powers give you access to powers mortals can only dream of. Whenever you consume a creature with Devourer of Death, you also inherit all their memories before their death and retain the ability to take the appearance of the consumed being permanently.

In addition, as a bonus action, you can mutate yourself and gain one of the following benefits:

• You can hurl your flesh and attract it back, granting melee weapons from your Armory of Mutation the Thrown (60/120) property for 1 minute.
• You grow a pair of eldritch wings, giving yourself a flying speed equal to your walking speed and can hover for 1 minute.
• You grow writhing cilia and gills, gaining a swimming speed equal to twice your walking speed and can breathe underwater for 1 hour.
• Your body, along with any equipment you are wearing or carrying, becomes pliable and fluid. For 1 minute, you can move through any space as narrow as 1 inch without squeezing and are under the Freedom of Movement spell.

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. You can also expend a use of your Second Wind to regain one use of this feature.

Nightmare Manifest

Your body enters its next stage of evolution. The weapons you wield now have the power to annihilate all who stand in your way. Once on each of your turns instead of taking the Attack action, you can strain your body to generate a more violent eldritch power with a weapon of your choice from Armory of Mutation. Targeted creatures must make a saving throw against your Living Nightmare save DC, taking damage equal to 5 hits of the chosen weapon, or half as much with a success:

Needler. Creatures of your choice within a 30-foot cone in front of you must succeed on a Dexterity saving throw. On a failure, they also take additional damage equal to your Constitution modifier (minimum of 1) each time when they take damage, until the end of your next turn.

Arm Club. You smash the ground below you, causing quakes and sending debris flying. Creatures of your choice within 30 feet of you must succeed on a Constitution saving throw. On a failure, they become restrained until the end of your next turn. If a creature fails the save by 5 or more, it also becomes unconscious until they take any damage or the end of your next turn (whichever comes first).

Lashing Tendon. You swipe those around you, disarming them of their equipment. Creatures of your choice within 30 feet of you must succeed on a Strength saving throw. On a failure, they also drop an object of your choice on their person. The objects land at their feet.

Sacrificial Bow. You assult them effortlessly with a flurry of arrows, draining their will and rendering them helpless. You target creatures of your choice within 120 feet of you equal to your Constitution modifier (minimum of 1). Each creature must succeed on a Charisma saving throw against your Living Nightmare save DC or take damage equal to 5 hits from your Bow and have disadvantage on attack rolls and saving throws until the end of your next turn.

You can also use this feature as part of a reaction to fortify your sinister aegis:

Sinister Aegis. You expand your shield to protect those around you. Instead of the normal effects of your shield, creatures of your choice within 30 feet of you (including yourself) gain a +5 bonus to AC and has resistance to all damage until the start of your next turn.

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. When you roll initiative, you regain one use of this feature.

Forbidden Corruption

You can temporarily infect creatures with the nightmare that lives within you, turning them into puppets. You always have the Dominate Monster spell prepared. With this feature, you can cast it without a spell slot or components, and your spellcasting ability for it is Constitution. You do not need to concentrate on this spell, and taking total and precise control of the target is a bonus action instead. Once a creature fails the initial saving throw, you cannot use this feature again until you finish a short or long rest.