A Dark Offering is a randomly occurring event in Lubenia, just like most of the events in the game. In this event, you can leave an offering on the Nameless God's altar in exchange for an item that you need more. Almost any item can be left as an offering: the more valuable your gift, the greater the chance that you will satisfy the Nameless God and receive a favorable exchange. You can only make one exchange per event.
If the item you want is not on the list below, then you must experiment with your offerings to win the Nameless God's favor.