Armour: for using this item the proficiency with heavy armour is needed.
The Demon Arm grants a +2 bonus to AC, resistance to fire and acid damage type, and +1 to CON characteristic. This item covers only an arm of the body so consider this item to cover only an arm slot and not a full armour.

Damnation command: at the magic word “Hellheimer!” the armour will turn in a full demonic arm with tentacles and claws spitting acid and fire. Consider the arm as a light secondary weapon. You can still add your STR modifier to this attack. The arm does a 1d6 of bludgeon and piercing damage +1d6 of Fire damage and +1d6 Acid damage. The effect last 3 rounds, for using the ability another time the wielder must make a long rest. The bonus and resistance won’t change during the demon form. -

Intrinsically evil: This piece of armor produces an evil aura making you detectable from spells like Detecting Evil.

Notes: Bonus: Armor Class, Bonus: Constitution Score, Resistance: Acid, Resistance: Fire