Weapon

Uncommon Requires Attunement

(Hand Crosssbow) (Requires attunement by a creature missing an arm, forearm, or hand)
  A ballista prosthesis is a magical weapon composed of numerous complex mechanical pieces that allow it to act as a functional prosthetic arm. You gain a +1 bonus to attack and damage rolls made with this magic weapon. This crossbow has an internal magazine that can hold up to 20 bolts. While there are bolts in the magazine, the crossbow does not have the loading property. It takes 1 minute to reload the magazine. The prosthesis attaches to your arm at the elbow or the shoulder, and it can't be removed against your will as long as you're attuned to it. To attune to this item, you must hold it against your arm for the entire attunement period. While attuned to the prosthesis, you can use an action to remove or reattach it. As a bonus action, you can retract the crossbow's limbs to the side of your hand or extend them from there. While retracted, the crossbow is concealed and functions identically to a normal hand. A creature can use an action to make a DC 13 Intelligence (Investigation) check, identifying its crossbow components and determining that it's a concealed weapon on a success. While extended, you can use the weapon as if you were holding it, and you can't use that hand for other purposes.