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 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. 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 the Bigger Bad of the 5th 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 Rumplestiltskin, meaning that it is indirectly responsible for the entire series.

Known Hosts

 * Nimue
 * Gorgon the Terrible
 * Zoso
 * Rumplestiltskin
 * Emma Swan
 * Captain Hook

Origin
The Darkness was a shapeless form of evil, presumably sentient, that took control of Nimue, Merlin's lover, as its first-ever host after she succumbed to its temptations, crushing the heart of the man who destroyed her home and family. Merlin was unable to finish it off, and because of its pure wickedness, he was forced to seal its might to the dagger - the broken tip of Excalibur. The human wielding the power would eventually become the Dark One. Nimue, under the Dark One's influence, binds Merlin within a tree after obtaining the dagger.

Eventually, Nimue was destroyed by a new future host, and the Darkness leapt into them, now using their body. After a series of unnamed hosts, Zoso became the Dark One and was later killed off by Rumplestiltskin, who became the most recent Dark One and sparked a series of terrible events in his long reign.

Season 4
Over the series, the Darkness has been darkening the heart of Mr. Gold, also known as 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".

Season 5
The Darkness is the Bigger Bad of Season 5. It takes the form of Rumplestiltskin, telling Emma that he's the manifestation of the Darkness, and the evil side of Rumple. Throughout the first few episodes, Darkness Rumple attempts to persuade Emma to embrace the role of the Dark One, trying to persuade her to crush the heart of Merida, and attempting to get her to steal the dagger back from Regina, whom Emma entrusted it with. Emma realizes, when she can be happy with Hook and focus on him, Rumple vanishes from her mind. When Merlin and Emma go to complete the sword Excalibur by retrieving the ember from the first Dark One (Nimue), Emma calls Nimue forth and the Darkness within her summons Nimue, who tries to persuade Emma to kill Merlin, until Emma snaps in rage and forcefully rips the ember from a stunned Nimue, who dissipates, saying she'll be waiting in Emma's mind.

The Darkness is not one to be undaunted so easily, however, and returns in Storybrooke (once again as Rumplestiltskin) even after Emma has accepted her role as Dark One, instructing her to build up a hero to retrieve Excalibur - the now-reformed Mr. Gold.

With Gold now a hero, Excalibur is within Emma's grasp. The Darkness within her mind summons not only Rumple and Nimue, but every other Dark One from the past to observe her completion of the ritual. She merges the dagger with the blade using the ember - Excalibur is whole again, and despite Merlin's warnings not to take the sword, after Rumple and Nimue urge her on, Emma grabs it out of the air as the Dark Ones around her smile in triumph.

It is revealed that the Dark One was also influencing Captain Hook, who was shown to be another Dark One at the end of Birth. The Darkness compels him to get revenge on Rumple, but Hook loses the ensuing duel. Nevertheless, he has the blood needed to let Nimue, the original host of the Darkness, to cross over, along with all other Dark Ones.

The Darkness is the ultimate antagonist and omnipresent force of evil in the Winter Finale, "Swan Song". It's power grows as all the Dark Ones, Nimue included, descend upon the town and begin their takeover. The heroes fight against the Darkness in a bid to save the town, and Hook desires to make Rumplestiltskin pay. The Darkness appears to be winning as Nimue begins to defeat Emma, but in the end, Hook decides to destroy Nimue and the others and sacrifice his own life to seal the Darkness away with him and all other Dark Ones. Instead, Gold performs a swap, believing his loved one Belle has given up on him, and allows the Darkness to seize him once again, giving him the power of every Dark One, including Emma and Hook, meaning that it got away with every action it performed this season and was not destroyed along with its hosts.

Trivia

 * The Darkness can be considered the Bigger Bad of the entire series, being the most powerful force of evil.
 * Merlin was unable to defeat it completely, merely binding it to the dagger after it took Nimue.
 * It is responsible for every Dark One (Gold included) throughout the franchise.
 * Its power was supposed to have vanished after Gold killed Peter Pan along with himself, however it continued to exist after Gold's resurrection.
 * Arguably, The Darkness is one of the two main antagonists of the series as whole, as it is behind many main antagonists.
 * It latched onto Rumplestiltskin, leading him to become evil and manipulative and eventually becoming the main antagonist of season 4.
 * Rumple, inhabited by the Darkness, was the one who trained Regina Mills in magic, while ultimately helped her become the Evil Queen and eventually the main antagonist of season 1.
 * Rumple was also behind Cora Mills (mother of Regina) learning magic, which led to her becoming as manipulative and evil as he was and the main antagonist in season 2a.
 * He trained Zelena as well, the main antagonist in 3b.
 * And in season 4, Rumple, inhabited by the Darkness, reassembles a trio of villains known as the Queens of Darkness, recruiting Ursula and Cruella De Vil and resurrecting Maleficent, the main antagonists in 4b.
 * However, it should be noted that he is not (as far as we know) responsible for Maleficent, Cruella, or Ursula becoming main antagonists in the first place.
 * In Season 5, the Darkness latches onto Emma Swan, who is this season's primary antagonist up until "Birth", when it was revealed that Emma forcibly turned Hook into the Dark One; therefore, Hook replaces her as the main antagonist in "Broken Heart" and "Swan Song". Towards the end of the latter episode, Hook absorbed the Darkness and sacrificed himself by telling Emma to kill him.
 * The first Dark One is revealed to be Nimue in "Nimue".