This is a vow to never forget the death of Zayn. The oath to get revenge is branded in to the prosthetic. 

The limb is complete golden to show gratitude to Zayn’s love for Gold. 

This item replaces a lost limb—a hand, an arm, a foot, a leg, or a similar body part. While the prosthetic is attached, it functions identically to the part it replaces. It can’t be removed against your will.

Notes: The prosthetic is complete made of mithral.