The Dark One's Curse, also sometimes known as the "Dark One Curse", is the frightening curse of possession dealing with an evil human becoming a Dark One host under the control of the wicked magical being known as The Darkness. It is one of many enchantments featured in the ABC television series Once Upon a Time.
Centuries ago, after the tragic woman Nimue who lost home village of Oxleigh to the warlord Vortigan, used her newfound magic as a sorceress from the Holy Grail for evil, the Darkness takes root inside of her and she becomes the first Dark One, being powerful enough to trap her mentor, the wizard Merlin. Nimue's cursed lives in all other new Dark Ones after she dies as these individuals would carry the burden being the vessel of the Darkness itself.
It is revealed that the Dark One was also influencing Captain Hook, who was shown to be another Dark One. The Darkness compels him to get revenge on Rumple, but Hook loses the ensuing duel. Nevertheless, he has the blood needed to allow Nimue, the original host of the Darkness, to cross over, along with all other Dark Ones from the Underworld.
The power of the Darkness grows as all the Dark Ones, including Nimue, descended upon the town of Storybrooke and began their hostile 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, Mr. Gold performs a swap with a potion to make the blade of Excalibur a magic "conduit", 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 Swan and Captain Hook, meaning that it got away with every action it performed this season and was not destroyed along with its hosts and it's Curse was not extinguished until years later when it destroyed by Weaver/Rumplestiltskin to save Wish Realm Hook and destroy the Wish Realm version of Rumplestiltskin who held part of the curse within himself.
The idea of the Dark One's Curse is that only one being can be cursed at a time. Nimue the original Dark One, created the Vault of the Dark One. After she created it, she emerged the vault. The Dark One was then killed with the Dark One's dagger. The position and power of Dark One then shifts to the person who stabbed the Dark One with that dagger, and so on. Zoso eventually kills the Gorgon the Invincible]] and becomes the Dark One himself. Down the track, he tricks Rumplestiltskin into killing him with the dagger, and therefore Rumplestiltskin becomes the Dark One.
When someone becomes the Dark One, they possess incredibly unlimited power. They are immortal, unless fatally wounded with the dagger or they are fatally injured in an area without magic, in which the Dark One's name would simply vanish from the dagger and this curse would end forever, which is what nearly happened in the case of Rumplestiltskin. If the Dark One stabs himself/herself with the dagger, then it is the same case and there is no new Dark One. However, the Dark One can be resurrected with the Vault of the Dark One.
Each Dark One's appearance and behavior change explicitly. Their skin turns grey and Their skin turns grey and scaly, their eyes become an animal shade of brown and their voice changes. As time goes by, the Dark One also exhibits and gains some murderous traits, becoming less merciful and more corrupted with the dark magic that is now flowing through their body. For example, Rumplestiltskin turned someone into a snail and crushed them, because that person accidentally and barely injured Rumplestiltskin's son Baelfire.
The Darkness actively used it's curse and gained complete control over a total of 25 people including least 18 unnamed Dark Ones and 7 certain Dark Ones:
- Nimue - The first Dark One.
- Gorgon the Invincible - Became the Dark One at one stage. He is a creature resembling a Bandersnatch.
- Zoso - Became the Dark One after Gorgon the Invincible, was controlled by the Duke.
- Emma Swan - Became the Dark One when the Darkness chose Emma as its next host, until it was expelled.
- Merlin - After being tethered to Excalibur, Merlin's Darkness grew within him.
- Killian Jones/Captain Hook - When Emma Swan tethers a dying Killian to Excalibur instead, until the Darkness was taken out.
- Rumplestilskin - The last Dark One alongside his Wish Realm counterpart, the Darkness temporarily expelled from him but eventually returned and was later destroyed by Rumplestiltskin himself.
- Wish Realm Rumplestiltskin - The last Dark One alongside his main counterpart having taken on a part of the Dark One's curse and the Darkness after he was created by the Evil Queen's wish.