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The Darkness is a powerful evil entity in the TV series, Once Upon a Time, 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 One's Dagger Dagger that would eventually become the Dark One's dagger, 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 Rumpelstiltskin and every other Dark One throughout the entire franchise.
The Darkness is the true and final antagonist of the 4th season, and can be considered the true and most dangerous villain of the entire franchise, given that Merlin, the most powerful known force of good, was unable to destroy it, and that it was the power behind every dark action taken by Rumpelstiltskin, meaning that it is indirectly responsible for the entire series.