The Darkness (Once Upon a Time)

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 to the soul of a human, as well as the 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. Thusly, the Darkness is indirectly responsible for every villainous action taken by Rumplestiltskin 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.

Season 4
Over the series, the Darkness has been darkening the heart of Mr. Gold, AKA Rumple, meaning that he will eventually die inside if he loses his last bit of love within, allowing the Darkness to have complete and utter control. In the finale episode of Season 4, Operation Mongoose, Merlin's apprentice is able to drive the Darkness out of Gold's heart, thus removing him as the Dark One once and for all. The apprentice attempts to suck the Darkness into Merlin's magical hat, but the evil is too strong and escapes as a dark swirling mist, starting the final battle in Season 4.

It first attacks the apprentice, attempting to possess him, but Emma Swan, the protagonist, attacks it with her powerful light magic, driving it out, at which it escapes out the door and into the town of Storybrooke. It next attempts to possess Regina Mills, but Emma, using the dagger, summons it to her instead, where it leaps upon her and binds itself to her instead, causing her to disappear along with it. The dagger drops to the ground, now possessing the name, "Emma Swan".

Trivia

 * The Darkness can be considered the Bigger Bad of the entire series, being the most powerful force of evil.