A major battle will erupt in the midseason finale of "Once Upon a Time" season 5.

After opting to save Hook's (Colin O'Donoghue) life in episode 10, Emma Swan (Jennifer Morrison) will have to face the effects of her decision in episode 11 titled "Swan Song," airing Sunday.

Based on the synopsis for the episode, all the past villains will be summoned by Hook to spread fear in Storybrooke with the help of the original Dark One, Nimue (Caroline Ford).

Because of the reappearance of all the former Dark Ones, Gold (Robert Carlyle) tells Emma's parents Snow White (Ginnifer Goodwin) and Prince Charming (Josh Dallas) that they should start savoring their last moments together. But Emma refused to give up, especially since she is the one to blame for dragging Hook to the dark path.

O'Donoghue revealed in his most recent interviews that he only learned about turning into a Dark One weeks before the filming of the episode.

"It was probably two or three weeks, if I recall, when [series creators] Eddy [Kitsis] and Adam [Horowitz] said, 'This is what's going to happen....' 'I was delighted to be able to explore that side of Hook again, as a dark, villainous character — only this time he's amplified by a thousand, you know?'" the actor revealed in an interview with TVLine.

Meanwhile, O'Donoghue told The Hollywood Reporter that Hook is very angry at Emma for turning him into a dark one because he knows that he does not have the willpower to control himself. However, he admits that Hook's anger does not mean that he wants to kill her.

But when asked if there is still a chance for Emma and Hook to patch things up if the darkness is lifted, the actor said that he is not sure that it will happen.

"The darkness is one of those things where I think if they're both dark, I don't know if that could work. At the minute with the way that Hook is, it's a very difficult thing to see happening ... it's complicated," the actor explained. "Much like most of their relationship, to be honest. It's pretty much a complicated thing."

"Once Upon a Time" season 5 midseason finale airs Sunday, Dec. 6, on ABC.