Casper is a somewhat eccentric and out-of-it person who was soul-forged with a certain white chicken near the citadels ruins. In speech, you can ask him several chicken-related questions where he is clearly unimpressed with your non-seriousness (after all, you weren't the one soul-forged with a chicken). When the WHITE chicken is killed, you will kill Casper and receive an achievement.

The Chicken can be found at the nearby Bellegar Shrine, after you talk to Casper about it.