I made this for "what if every franchise existed at once in the same universe"The magic prosthetic is a common magical item that seems to be made from a stone according to tests granite. The way the prosthetic works is that one places a rune in the slot on the back and then can use the properties of the rune

Runes

Weapons rune

This rune allows the user to summon weaponry from an unknown place or power up the wielders existing weaponry