Automail

A high grade prosthetic given to generals and field marshals, this magical item is intended for utility but is a dangerous weapon in its own right. In its inert state, this prosthetic would attach to the upper arm of most humanoids and is made of interlocking layers of dark metallic material. When worn and attuned, the arm will change and form to cover any remaining arm along with the shoulder almost up to the neck and wrap around just under the other arm. The arm then comes to life, reacting as naturally and seamlessly as a flesh and blood arm and is as comfortable as wearing nothing. It even allows for a certain amount of tactile feedback, being able to feel pressure and a certain amount of heat and cold. 

This arm is monstrously strong and fast, aiding the user in many ways. Any strength and dexterity related things that can be done with the arm become easier. This makes climbing a trivial task, and aids in strength based attacks. Its hardened metallic nature provides excellent protection against weapons and allows shields to be worn by being magically magnetically attached to the forearm. Its can be used as a spell-casting focus and any ranged spell attacks have double the range. Using its sheer strength the prosthetic can be used as a weapon itself, its punches landing like a battering ram moving at incredible speed.

You are proficient with this prosthetic. 

• Proficiency: The arm provides proficiency with (Athletics) and (Sleight of Hand).
• Movement: You gain a climbing speed of 30.  
• Strength bonus: You gain a plus 1 to strength based attacks.
• AC bonus: You gain a plus 2 to your AC.
• Shield master: You gain proficiency with shields and wielding a shield does not require a free hand.
• Spell casting: The arm is a spell-casting focus and doubles the range of any spells that require an attack roll. 
• Bonus attack: The arm can attack for 1d8 bludgeoning damage as a bonus action without needing the attack action and suffers no penalty for an offhand attack. 
• Attack action: The arm can deal 1d10 bludgeoning damage when used to attack as an action. 
• Siege: The arm deals double damage to objects and structures.

Proficiency with a Warhammer allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Proficiency: Sleight of Hand, Proficiency: Athletics, Bonus: Armor Class, Ignore Weapon Property: Heavy, Weapon Property: Light, Bonus: Spell Attack Range Multiplier, Ignore Weapon Property: Two-Handed, Set: Innate Speed (Climbing), Bonus: Magic, Proficiency: Shields, Bonus: Dexterity Saving Throws, Bonus: Strength Saving Throws, Bonus: Strength Attacks, Weapon Property: Finesse, Ignore: Dual Wield Modifier Restrictions, Proficiency: Warhammer, person with a missing arm, Damage, Combat, Versatile, Push