It is an evil force that was battled by Merlin the Sorcerer centuries ago. Unable to destroy it, Merlin bound the powerful evil to the soul of a human, as well as the Dark One's Dagger that would eventually become the chosen Dark One's very cursed weapon, allowing the possessor of the dagger to control the evil human. Upon the Dark One being killed by the dagger, the Darkness would then possess the killer, making them the new Dark One and branding their name onto the dagger. Thus, the Darkness is indirectly responsible for every villainous action taken by Rumplestiltskin and controls every Dark One throughout the entire franchise.