Adamantine Arm Prosthetic/Exoskeleton

This device is attached onto an existing arm or replaces a missing arm. You can detach or reattach it as an action, and it can't be removed against your will. It detaches if you die.

Armored Shell. Gain a +2 to AC while not holding a shield. You can only gain this bonus to AC from one exoskeleton. 

Door Knocker. When you make an unarmed strike using this arm, the attack deals an additional 1d4 Bludgeoning damage, or an additional 2d4 Bludgeoning against constructs. Additionally, unarmed strikes you make using this arm against an object that hit are automatically critical hits. 

Hefty Design. The exoskeleton's material causes it to weigh more than usual. If your Strength score is below 15, your speed is reduced by 5 feet.