The Unusual Artifact is the Bundle carried by Paulus the Abbot during the A Monk's Mission quest done in The Catacombs. If you let Paulus die when he is coming back to the entrance of The Catacombs after gathering the artifact, he will drop this artifact. Other than for the quest, this item has no other known uses in the game.
Quest Involved in
This item will not appear on the floor when Paulus is going to take it. Consequently, the only way to obtain this item is to let him slowly die by the effect of the Swamp Water.