Chome had a bad day and the backfire from a misused ingrediant resulted in the loss of his right arm. Chome is a positive person and wasn’t going to let this upset him so he set out to create a replacement.
He started trying to make a physical arm out of wood, metal, even plants at one point but the result was thoroughly unsatisfying. During all his experimentation he became an expert on animating the inanimate and became very successful through this expertise.
Finally through all his trials he devised a phantasmal force that can be manipulated exactly as if it was his arm. The arm is invisible but it is a physical manifestation of a force. The phantom arm can be used exactly how the old arm would be through mental and muscle cues. As long as the necklace is worn it is like he was never missing an arm. He wears long shirts and gloves and you would never know it was missing.
There may be other similiar items in circulation made out of just about any type of material you can think of created during his search for the perfect replacement arm.
The arm doesn’t inbue any extra strength, if used by a fighter they would probably find the arm under powered. The arm is resistant to cold, heat, etc., and does not relay any pain that may normally be felt by it.
It is a physical manifestation so that it cannot go through walls and such. Because it is a phantom extension off the body, it can hold cursed items and such and not be affected by them, on the same note, most magical weapons may not work if used by that hand if contact with the wielder is required for their power.