With darkness looming, Hook (Colin O'Donoghue) and Emma (Jennifer Morrison) will face off in the upcoming midseason finale of ABC's Once Upon a Time.

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Hook has completely turned into the Dark One and will be in the process of damning a soul and opening up a portal to Hell in the midseason finale, titled "Swan Song." With our heroes in Storybrooke now more unsafe than ever, Emma will try her best to undo her past destruction as Hook grows more evil.

ABC has released a plot synopsis for the episode, teasing that the two characters will face off in an epic battle:

"The dark siege of Storybrooke is underway as all of the resurrected Dark Ones target a living soul for sacrifice so that they can return to the realm of the living. With doom imminent, Gold advises Mary Margaret, David and the rest of the heroes to spend their last fleeting moments enjoying the company of their loved ones," the synopsis reads. "Emma, however, refuses to give up, knowing that she is responsible for righting all the poor choices she made as Dark One - there's only one problem, Dark Hook. When they face-off with the fate of Storybrooke at stake, Hook's actions will devastate Emma in a way no one saw coming."

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There will also be a flashback sequence in which the Evil Queen (Lana Parrilla) "tests Hook's mettle by forcing him to confront a demon from his past before contracting him to dispatch one from hers."

It is sure to be a devastating episode to leave people hanging until March, but it sounds like this may be the most epic episode of Once Upon a Time yet.

Once Upon a Time airs Sundays at 8 p.m. ET/PT on ABC.

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Once Upon A Time, Television, ABC