Alright. This started with a basic prosthetic. Electrical signals, which power a typical prosthetic, will not work here, as the nerves in the body are dead. Those are replaced with Unholy energies, of sorts, that allow control of the body. The easiest way, if you want mechanical, is to power it with Unholy Runic magic. This can't be done without a Techno-insert-class-here, which makes this out of the question, as there are none of those on the server. You could have some sort of a battery built, then, to replace the Unholy energy, which would solve the issue of power, but then there's the problem of connecting it to be powered by your will... This could be solved with some sort of direct electrical link to the brain, but I'm not 100% certain if this would succeed. This brings us to the laat real possibility, in my mind: pneumatics. Powered by pressurized oil, you can make this thing move at varying speeds, with varying amounts of power. It will never be as powerful as his actual arm, however. This still leaves us with the issue of wiring a connection directly to one's brain, so it can be powered.