When Once Upon a Time’s 100th episode airs on March 6, things will be very different, yet eerily similar. As co-creators Adam Horowitz and Eddy Kitsis teased to The Hollywood Reporter earlier this season, the Underworld is a twisted version of Storybrooke, filled with dearly (and not-so-dearly) departed characters from the show’s five seasons.

According to the fairy-tale drama’s core cast — Ginnifer Goodwin (Mary Margaret/Snow White), Jennifer Morrison (Emma Swan), Josh Dallas (David/Prince Charming) and Lana Parrilla (Regina/the Evil Queen) — there are going to be many ghosts with “unfinished business” from the past, sending everyone on an emotional journey.

The person everyone is most worried about is Emma, who Morrison says is “vulnerable” and in a “deep, dark place” during her quest to find Hook (Colin O'Donoghue). Find out how far her parents and Regina — who Parrilla says is now “family” — will go to save Hook, what shocking reunions are ahead and what other surprises can be expected from the milestone episode.

Underworld Makeovers

Just like in other realm shifts on OUAT, the Underworld is going to change everyone. But, according to Morrison, Emma may be the most affected.

“Emma is really leaning into the fact that she truly, deeply loves Hook and that she's going to risk everything to try and save him,” Morrison tells THR. “We're going to this deep, dark place, and she's going there more full of love and hope and vulnerability than she's ever been before. … I think her having gone through the "Dark Swan" phase has given her the opportunity to be that open and vulnerable, and it's almost like looking at her own inner demons in a certain way in order to get to a more open, free place and the other side of it.”

Now that the Savior is preoccupied, Regina also will become a hero in the Underworld, helping lost souls complete their unfinished business.

“Because Regina is now a hero and has joined the hero pack, she feels obligated to help these souls. … It's giving everyone the opportunity to make amends with their past and help these souls move on,” reveals Parrilla. “And that's the hero in all these characters. We're not just down there to help one person — we end up going down there to help many.”

One of the biggest pieces of unfinished business happens to be with Regina and her mother, Cora (Barbara Hershey). “When she sees her mom for the first time, it's a happy reunion, but it's bittersweet,” says Parrilla. “We start to learn why Cora's trapped down there, and we learn that Cora's there because Regina is part of her unfinished business.”

Emotional Reunions

Regina and Cora’s bittersweet reunion is just one of the big returns the 100th episode has in store — and some of them will be even more shocking than the evil family’s.

“There are going to be some interesting meet-ups. … With our characters meeting up with parts of their past that they may or may not want to come across,” teases Dallas. His on- and offscreen wife, Goodwin, agrees. "The Underworld is less a setting and more a character in and of itself [that] affects each of us deeply," she says.

"We're faced with not only the people who are trapped with unfinished business, but the realizations that we might be responsible for that, in some way."

Snow White’s biggest reunion (and one of Goodwin’s favorite scenes of the series) will be with “someone from her past that defined who she was,” she says. However, she teases, it’s “not someone we've met before — it's a new character,” who will appear alongside Bailee Madison’s young Snow White. Meanwhile, Charming also will come across a family member whom he’s never met before, but who's "always been a very big part of his life."

Morrison didn’t reveal exactly whom Emma will see in the Underworld — although Michael Raymond-James is returning as her ex, Neal, for the first time in two seasons — but she hinted at “several fun surprises and unexpected encounters” as they try to find everyone happy endings.

Baiting Hook

Snow White and Charming are Emma’s “hero team” first and parents second in the Underworld. Dallas says that since the couple has “gone to great lengths to save each other” and literally share a heart, Charming understands how important it is for a person to find love. “He’s going to give [Emma] the room to make her own decisions about [Hook] and whatever she needs to do to find that for herself,” he says.

After five years of a tense back-and-forth relationship, Regina also is going to give Emma her all. “She feels obligated for many reasons and responsible to help Emma because Emma sacrificed her life to save Regina,” says Parrilla. “Regina is a team player. She is part of this family, and family is everything to her, so she is going to help Emma as far as Emma wants to go to save Hook.”

Once Upon a Time’s 100th episode airs on Sunday at 8 p.m. on ABC.

—Reporting by Kate Stanhope