Bad news: Emma goes dark. Good news: Emma goes dark.

When Once Upon a Time returns on Sunday (8/7c, ABC), Emma Swan (Jennifer Morrison) made the ultimate sacrifice and now is trying to resist becoming the Dark One, or in her case, the Dark Swan. She's the Savior after all, and giving into the toxic urges of dark magic won't be pretty for her loved ones. As viewers, we don't want that either, except that her Dark Swan persona looks absolutely badass and kind of fun.

"We're going to be seeing her whole struggle as she moves toward darkness," co-creator Adam Horowitz told reporters at a screening. Co-creator Edward Kitsis added, "What's the most exciting part of Rumplestiltskin's backstory is finding out how he went completely dark, and so for Emma, whether or not she goes dark, we want to see that struggle. A character as strong as Emma, can she resist it?"

Once sneak peek: Emma faces off with Merida!

This also means that we'll be delving into the whole Dark Ones mythology and the surprising origins of the dagger that controls them. Meanwhile, Emma's friends and family learned that the wizard Merlin is the key to saving her, so they're headed to Camelot.

Check out what else we can expect this season on Once Upon a Time:

Hooked on a feeling Poor Hook (Colin O'Donoghue). Emma finally said, "I love you," before she got whisked away by the Dark One magic to who-knows-where. "He doesn't feel good about it," Kitsis tells TVGuide.com. "This is a man that when he has a task he doesn't lose. He spent 300 years trying to get revenge, so now that Emma's the Dark One, nothing is going to stop him from saving the woman he loves."

Queen of guilt Regina (Lana Parrilla) was actually the target of the dagger, but Emma took her place, which now means Regina is duty-bound to be a savior. "At the beginning of last year... Emma said, 'I promised to bring back everybody's happy endings, including yours,'" Kitsis says. "So at the end of the year, we watched Emma sacrifice herself for Regina's happiness. If you know Regina, somebody sacrificing themselves for you, and you owe them, pisses you off. You're going to get even, no matter what."

A bun in the coven Regina's half-sister Zelena (Rebecca Mader), aka the Wicked Witch, is currently locked up in Storybrooke after it was revealed that she killed Robin's (Sean Maguire) wife, magically masqueraded as her and got pregnant with Robin's baby. Awkward all around since this is also Regina's true love. "She's the Hannibal Lecter of Storybrooke," Kitsis says, referring to her notorious jailed state. Horowitz adds, "We are exploring the pregnancy. It's going to complicate things for Regina and Robin." Too late!

Trouble with the Charmings According to Kitsis, David (Josh Dallas) will have a "bromance like no other" with King Arthur and will become a "reawakened hero" in Camelot. All is not well with his wife Mary Margaret (Ginnifer Goodwin), however. "Mary Margaret and David have a past with someone from Camelot... and David is going to become pretty close with Arthur. Mary Margaret, if we remember back to Season 2, was able to have a child because of Lancelot," Kitsis says. "So there's gonna be a bit of palace intrigue. When you're married and there's a really bad situation and you can't do anything about it, sometimes you turn on each other. We're going to see Snow and Charming fight in a way we've never seen before, and we hope that the fact that they share a heart wins out."

Henry will get a quest (and romance!) on Once

Every rose has its thorn Belle (Emilie de Ravin) and Gold's (Robert Carlyle) love story has hit yet another rough patch after he was placed in a coma/deep sleep in last season's finale after getting rid of his darkness. "I don't think it'd be much fun to see him lying around the whole season," Kitsis says. "But I will say his arc in Storybrooke is something we haven't fully seen before. It's going to go right into his very essence as a character." As for Belle, her feelings are a bit mixed after the man she loved tricked her, but for now, she doesn't want him to die. Also, expect to see an iconic image from Disney's Beauty and the Beast in relation to their story.

Playing with legends As previously reported, the gang will attend a ball in Camelot. Horowitz adds, "There's nods to many things, including Sword in the Stone, alluding to the general King Arthur tale. We've dipped our toe into as many different facets of the mythology from the Disney version to the more classical versions." Kitsis says, "Arthur and his love for Guinevere is central to some of the conflict." Backstories on most of the main players in Arthurian legend — including Arthur, Guinevere, Merlin and Lancelot (Liam Garrigan, Joana Metrass, Elliot Knight and Sinqua Walls )-- will also play out.

Putting on a Brave face As we've already seen from this sneak peek, Emma and Merida (Amy Manson) go mano a mano in the premiere, which does not bode well for either of them. Shouldn't they be allies? "She's going to have a very special relationship with Emma and ... Rumplestlitskin," Kitsis says. "You realize that Merida's problem [is] she punches first and asks questions second. We're going to find her in the middle of a very intense situation. We're definitely going to explore her story more. She's going to get her own hour. We're going to go to Dunbroch and see the dad and the mom and everything. Flashbacks!"

The return of Mulan and Ruby Horowitz says, "First, we're not just planning to drop them back in the show with no explanation and just say, 'Oh, they were over in the corner.' We do want to give a taste of where Mulan has been and what she's been up to and what she's been going through. The same with Red." As previously reported, an LGBT romance is coming to the show, but it may or may not involve Mulan (Jamie Chung). As for Ruby (Meghan Ory) and her Granny (Beverley Elliott), you can expect them to wolf-out at some point. "Look, we live in a world with wolves and we want to be reflective of that," jokes Horowitz.

Henry's helping hand Emma's son gets his own romance this season (finally!) but he'll also be helping out. In fact, Hook will seek him out in the premiere in a risky scheme. "Hook and Henry have a lot of stuff that's coming up," Hororwitz promises. "Now that Emma has been pulled out of the center of the hero group, it shifts the dynamic in a way that does allow for [pairings] like Henry and Hook."

Heigh-ho! It's off to Camelot we go That's right, Grumpy (Lee Arenberg) and some of the characters who've often been left behind will get to have fun this year. Kitsis explains, "We just loved the idea of how annoying it would be to be a dwarf and you were the guard in the pilot, and now there's all these adventures and everyone's like, 'What was Neverland like? Oh, you didn't go? I thought you were Snow White's guard?' We just loved the idea of Grumpy being upset about it and feeling jealous."

Lily's quest on the backburner Unfortunately, Lily (Agnes Bruckner) isn't a priority right now, so her search for her dad will get sidelined for the near future. But don't expect to see a familiar face as her father. "It may be someone completely new," Kitsis says. "Lily was born an egg so how dragons are made is God knows right now."

Once Upon a Time premieres Sunday at 8/7c on ABC.

VIDEO: Once Upon a Time stars share their characters' ultimate happy endings