Get ready for an epic, two-hour Once Upon a Time.

During the show's first hour on Sunday at 8/7c on ABC, Emma Swan (Jennifer Morrison) may look fantastic in full Dark Swan/Lady Stardust mode, but her loved ones are afraid about what it means for her battle with the darkness inside her. When last we saw Emma, she had succeeded in finally mending Excalibur and, ignoring Merlin's (Elliot Knight) advice, claimed the sword as her own.

As you can see from the sneak peek above, wielding Excalibur might not be all that bad if it's going to save the life of her true love, Hook (Colin O'Donoghue). But who will save Emma from her dark side?

"Hook's dilemma is saving Emma," co-creator Edward Kitsis tells "She now has everything she needs to get the darkness out of her, and yet we see in Storybrooke, she is completely Lady Stardust-ed out, she's totally Dark One-ed out. So, we're wondering, if she has everything she needs to get the darkness out of her, what changed?"

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Check out what else Kitsis and co-creator Adam Horowitz had to say about Once Upon a Time's two-hour event on Sunday:

In the episode description, Emma is forced to make a "gut-wrenching" choice. What is the fallout of that choice?
Edward Kitsis:
For us, Emma's heart-wrenching decision ... really comes into play in the way that will make you go back and kind of look at the season in a new light.
Adam Horowitz:
That heart-wrenching decision kind of amps everything up and starts the ball rolling down the hill toward the midseason finale. What Emma does or doesn't do this Sunday is going to have long-reaching ramifications.

Colin O'Donoghue and Jennifer Morrison,<em> Once Upon a Time</em>Colin O'Donoghue and Jennifer Morrison, Once Upon a Time

We've also learned that Zelena's (Rebecca Mader) pregnancy "accelerates." What's causing it?
Well the question is, "Who did it and why?" Was it Zelena because she's out of her mind and completely wicked? Or did someone do it to her for a different purpose? I can tell you it will be answered in the first hour this weekend.

Accelerated gestation does not sound healthy or normal.
Well, it does sound magical.
I will say that nothing with Zelena is healthy or normal. She is a very wicked character but she is also a human character, and this pregnancy is affecting her on a deeply human level despite all the terrible things she's done. Now we're going to see how her facing that humanity that's literally growing inside her, and what is inside her, affects her and what it makes her do, and as things tend to be with Zelena, her reactions are usually wildly unpredictable.

Liam Garrigan and Rebecca Mader, <em>Once Upon a Time</em>Liam Garrigan and Rebecca Mader, Once Upon a Time

What is coming up for Henry (Jared Gilmore) now that he's been the victim of Emma's darkness?
He had his heart broken, and like any teenager, he got sidelined by his first love. But we're going to see some serious hero-ing coming up from Henry.

Does the hero-ing have to do with him being The Author?
It's less about him being The Author and more about him being a part of a very dysfunctional family.

Why are Regina (Lana Parrilla) and her friends tied to trees in this episode?
Our characters, while they're in Camelot, find themselves in a sticky situation.
And we know that Merlin has been used against them. So, you know they're facing a very formidable foe in that Arthur now has his own equivalent to the Dark One. That's going to go down pretty hard on Sunday.

<em>Once Upon a Time</em>Once Upon a Time

What can you say about the return of Dr. Whale (David Anders)?
He delivers one of my favorite lines of the season. And when you see him, he'll have a very striking resemblance to someone else on our show.

In the second hour, Merida (Amy Manson) discovers what happened to her father King Fergus (Glenn Keogh) when he died in battle. How will Mulan (Jamie Chung) help her?
As you have found out, they have a past history together, and Merida is going to find some trouble in her kingdom where she's going to need to rely upon an old friend for help, which is Mulan. Through that hour, we're going to find out where Mulan has been this past year as well as where Ruby (Meghan Ory) has been.

How does Ruby fit into their trio? Does her being a wolf come in handy?
You can't have Ruby and not have a wolf be handy. That's like going to get Batman and having him forget his utility belt. The three of them together team up to solve a problem in a really pretty awesome way. [It's a] fun, very Once Upon a Time adventure, with three strong women kicking ass.

Meghan Ory and Jamie Chung, <em>Once Upon a Time</em>Meghan Ory and Jamie Chung, Once Upon a Time

What can you tease about the debt that King Fergus owed to a witch?
I can't tell you what it is, but I can tell you it's the witch from the movie Brave. That witch comes back to collect a debt from her father, and Merida has been tasked with paying it back. That is what she enlists Mulan's help in.

Does the witch rhyme like she did in the film Brave?
I don't know if she's as rhyming on this one, but she is formidable, I can tell you that. And I can tell you she has a great witch-out scene with the Wicked Witch, so you get to see witch vs. witch.

Many people believe that the etymology for the name Arthur is Welsh for "bear king." Does the title of the second hour, "The Bear King," refer to King Arthur (Liam Garrigan)?
Bear King refers to King Fergus, Merida's father.

Knowing Brave's connection to bears, is Arthur involved with Fergus' past?
Perhaps there is a connection.

Glenn Keogh, <em>Once Upon a Time</em>Glenn Keogh, Once Upon a Time

What sort of action will we see in the flashback battle scene, where Fergus dies?
This episode is the closest we've come to a Braveheart scene. We have a huge, huge battle with two armies.

Once Upon a Time's two-hour event begins Sunday at 8/7c on ABC.