The White Chicken that was soul forged to Casper is found in the Bellegar Shrine south of Casper himself. Once you find it you can either kill it, which in turn will kill Casper because of the soul forgery, or leave it alone. Unfortunately, this quest was never properly implemented; there is no way to help Casper. You can neither bring the chicken to him nor can you remove the soul forging. However, if you are playing the Xbox 360 version of the game, you can kill the chicken, murdering Casper in the process, and receive an Achievement.