'Once Upon a Time': 7 Things to Know About the 100th Episode

One hundred episodes is a major milestone for any television series, and Once Upon a Time is celebrating its achievement in a big way by bringing back familiar faces, introducing a new realm, and raising the life-and-death stakes (with emphasis on death).

Creators Edward Kitsis and Adam Horowitz have said they hope the 100th episode feels like a return to the first season. “In going to the underworld, all of our characters are, in many ways, forced to look forward in a way that was similar to Season 1,” Kitsis tells Yahoo TV.

Related: Ask the Fans: ‘Once Upon a Time’ Creators Want Your Answers to These 7 Questions!

The episode, titled “Souls of the Departed,” picks up where the winter finale left off. After Emma (Jennifer Morrison) was forced to kill her love, Captain Hook (Colin O’Donoghue), she leads her friends and family into the Underworld to save him. As is often true on OUAT, their journey won’t be easy — physically, magically, or emotionally.

Here’s what else we’ve learned about the 100th episode:

1. The whole gang is getting back together

The not-so-secret news is out of the bag — a whole host of (dead) characters are making cameos, since this is the Underworld after all. They include: Regina’s parents, Cora (Barbara Hershey) and Prince Henry (Tony Perez); Sidney Glass (Giancarlo Esposito); Peter Pan (Robbie Kay); and to fans’ delight, Neal/Baelfire (Michael Raymond-James).

Oh, and they won’t be the only familiar faces. Later this season we’ll see the returns of Mulan, Ruby, Cruella de Vil, Merida, Arthur, Guinevere, and Zelena.

2. The storyline isn’t just for the fans

OUAT fans are always vocal about which characters they want to see again, be it in the present storyline or flashbacks, but this isn’t just a one-off episode to give them a treat.

“For the 100th episode, as writers, we wanted to honor everything that has come before and find a way to acknowledge so much of what the show has been,” Kitsis says. “And yes, we always do it for the fans, but we also told the story in a very conscious way this season to get to this point. It’s not like we’re doing a detour to give curtain calls to characters or certain things. It’s about a season-long arc we’re playing with here.”

3. The blasts from the past are all about the future

Kitsis and Horowitz have said that their version of the Underworld is a sort of purgatory, not hell itself. Its residents are there because they haven’t found closure. How to find that closure is going to be the big question that the characters must address in the Underworld.

“We’re exploring the idea of unfinished business — things that were left unsaid, left undone, things in our past we wish we could correct, things in our past we wish we could go back and do,” Kitsis says. “I think a lot of these characters have a past that haunts them and prevents them from moving forward.”

Related: ‘Once’ Turns 100: Rebecca Mader’s Photo Diary

4. The Underworld is a twisted version of Storybrooke

Not only will the realm be populated by familiar faces, the actual locations will be familiar, too. Like, there’s Mr. Gold’s shop… except not. And a really weird Granny’s diner.

“There’s a very specific reason why the Underworld appears as a screwed-up version of Storybrooke,” Horowitz teased to TV Line.

5. Remember what they say about the devil you don’t know

A new world means a new big bad, and that’s Hades, the ruler of the Underworld, played by Greg Germann. And the actor relishes a role where “anything goes,” as he told Entertainment Weekly.

“The creators bring him to life on the page with the same out-of-the-box unpredictability that they bring to the rest of the show. They strike the perfect balance between keeping things grounded and letting things fly — sometimes literally.”

6. Finding Hook might be Emma’s most difficult task yet

Storybrooke’s Savior has gone through a lot, and faced a multitude of villains. But all those souls in the Underworld will be a major obstacle for her. Let’s hope she acts fast, though, as Hook isn’t doing too well there. “He’s suffering in the underworld, he’s suffering really badly,” O’Donoghue told ET.

7. One hundred down, one hundred more to go…

Considering how epic the 100th episode is, how will Kitsis and Horowitz top themselves with the 200th episode?

“We will let you know in July at Comic-Con!” they joke. “If we get lucky to get there, we’ll tell you then.”

The “Souls of the Departed” episode of Once Upon a Time airs Sunday, March 6 at 8 p.m. on ABC.