Arm of the Archdevil

 This Arm was ripped off the Archdevil Asmodius during his and Erythnul's fight being torn from the Archdevil by Erythnul who than beat him to death with it before turning his body into magic items like this one

These magic items were than rarely given out to those Erythnul found interesting or as rewards for his warlocks who pleased him greatly

Attuning to the Arm. To attune to the Arm, you must be gifted it by Erythnul himself , after attuning to the Arm it will apper as a tatto on the user's hand while not active , while active the Arm will change the user's arm from the elbow down into a demonic spiked arm ending in clawed fingers 

Properties of the Arm.(passive) When you are attuned to the Arm and it is not active you gain the following benefits:

Great Strength. Your Strength becomes 20 unless it is already 20 or higher.

Danger Sense. You have Advantage on Initiative rolls.

Poison Immunity. You have Immunity to Poison damage and the Poisoned condition.

Spellcasting. The hand has 10 charges and regains 1d4 + 4 expended charges daily at dawn. You can cast a spell on the Arm of the Archdevil Spells table from the hand (save DC 18). The table indicates how many charges you must expend to cast the spell. Each time you cast a spell from it, the Arm casts Suggestion on you (save DC 16; no Concentration required), demanding that you commit an evil act. The Arm might have a specific act in mind or leave it up to you.

Activation The Arm can be activated once per long rest or once per short rest as well if you are a warlock sworn to Erythnul

Properties of the arm (active) While attuned to both the Arm and it is active you gain the following additional benefits:

Firey Touch. Any unarmed attack deals an extra 2d8 fire damage on a hit.

Regeneration. If you start your turn with at least 1 Hit Point, you regain 1d8+con Hit Points.

Hellish resistance While the arm is active you gain immunity to all non magical damge

Removing the Arm The Arm can be given up freely Erythnul won't make someone who doesn't want his gift to keep it but don't expect it back unless you're willing to pay the price

Notes: Set: Strength Score, Immunity: Poison, Immunity: Poisoned, Advantage: Initiative, Damage: Fire, The 11th charge can only be used to activate the Arm of the Archdevil and recharges with a long rest