The Dark Key is an item used alongside a Light Key to open the Chest of Hope in the Dark and Light Minigame for a chance at the Yang and Yin Sets as well as the Godlike Aura.
Source[]
Drops from Mini God of Yin at a rate of 1/15.
Drops from God of Yang and God of Yin, each having a 1/2 droprate for either key.