Once Upon a Time welcomes you to the Underworld for its winter premiere and 100th episode Sunday. Emma (Jennifer Morrison) and her merry band of heroes travel to Once's version of Purgatory on a mission to find a dearly departed Hook (Colin O'Donoghue) in hopes of bringing him back to the world of the living.

The idea was to find Hook, give him half of Emma's heart and get out of there, but predictably the situation becomes a lot more complicated once the group arrives in the Underworld, which is set like a dark version of their hometown Storybrooke. The Underworld is populated with all the departed souls that have unfinished business and most of it is with the group of heroes that just strolled down Main Street. Living up to their titles, the heroes decide that along with saving Hook, they will attempt to free all of the souls from the Underworld and send them to a better place.

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It's a noble mission, but the truth is that confronting old ghosts has the potential to throw any of the heroes off track. Who is most likely to throw the mission off course because they're wrapped up in their own demons? Regina (Lana Parrilla) already gets a taste of it when she comes face to face with her father. Emma loses all color in her face when she realizes that a "Remember that time you pushed me off a cliff?" conversation with Cruella De Vil (Victoria Smurfit) is inevitable. Snow White (Ginnifer Goodwin) will have to have a similar conversation with Cora (Barbara Hershey). There's equal opportunity for a thief like Robin Hood (Sean Maguire) to run into someone he's robbed. Even Henry (Jared Gilmore) has the potential for conflict since he broke the quill used for authoring the story.

"Out of these heroes, everyone pretty much stays on target. When we are on a mission, sometimes we get thrown off track," Parrilla tells TVGuide.com. "That happens quite a bit with characters that come in."

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However, there's one person who always tends to lead the group in the opposite of its intended direction: Mr. Gold (Robert Caryle). The winter premiere has a small family reunion between Mr. Gold and his father Peter Pan (Robbie Kay). Pan is already trying to make a deal to trade places with one of Mr. Gold's travel companions so he can rejoin the living. Parrilla admits that it is in fact Mr. Gold who has the most potential to ruin the mission.

"We have Cruella De Vil, who makes an appearance. Giancarlo Esposito [is back]. Emma Caulfield is the Blind Witch," she says, "but the person who really tries to take us in a different direction is Mr. Gold. He always has his own agenda."

The question remains: Will Gold manipulate the group to help his father or for his own ulterior motive he has yet to reveal? We know he loves power and soon he'll realize Hades (Greg Germann) has more of it than any villain the group has seen so far.

What do you think about Mr. Gold's intentions in the Underworld?

Once Upon a Time airs Sundays at 8/7c on ABC.