A trip to the Underworld. Many returning characters. An emotional, cathartic confrontation. These are only a few things to look forward to when Once Upon a Time airs its 100th episode.

Josh Dallas, Sean Maguire, Jennifer Morrison, Jared Gilmore, Ginnifer Goodwin and Lana Parrilla on Once Upon a Time

Barbara Hershey on Once Upon a Time

Lana Parrilla on Once Upon a Time

Jared Gilmore on Once Upon a Time

Hitting episode 100 is a big moment for any TV show, and OUAT is using the opportunity to dive into its own history to trot out some familiar faces for its Season 5 midseason premiere. But beyond just calling back to previously lost characters, the episode is going to hearken back to the feel of Season 1."In Season 1, we the audience got to see Storybrooke through Emma's eyes as she drove into town and we had never seen Storybrooke before. In the 100th episode, we're seeing a different version of Storybrooke. Behind the scenes we call it 'Underbrooke,'" executive producer Steve Pearlman explained at OUAT's 100th episode party."It looks very much like Storybrooke, but everything is slightly different. The main street is different, every one of our sets and locations we go into is slightly different, and so from a production perspective it's been really fun to try to figure out what are the little things we can do to change. There's a lot of Easter eggs there fans are going to be able to find and enjoy."There are many returning guest stars as Once heads to the Underworld. There the core cast finds that familiar faces like Robbie Kay's Peter Pan, Barbara Hershey's Cora and Emma Caulfield's Blind Witch have taken up doppelgänger-type residence in Underbrooke. Cora lives in Regina's mansion, for instance, and the Blind Witch runs Granny's Diner -- except she serves children along with the normal diner dishes.These people make appearances in episode 100, but because Once Upon a Time is spending much of the second half of Season 5 in the Underworld as Emma & Co. try to save Hook, these characters -- and some others -- will stick around for a little while."I'm really excited for them to get to see some of their old friends return," said showrunner Eddy Kitsis. "I think that's what's really fun about these, is this episode is really to kind of honor the five years that have been. That's what we're most happy about."Showrunner Adam Horowitz added, "A lot of them are coming back into play, and a lot we'll touch on to some extent."Hershey is back for a new arc as Cora, with Caulfield returning for about five episodes. Victora Smurfit's Cruella de Vil reappears in episode 101 (though she does get teased in the 100th episode), and Kay is back for two or three episodes as Peter Pan. These aren't the only familiar faces who will be seen in the Underworld; Pearlman promises there will "definitely be a couple" surprises."Honestly I'm excited for everybody that comes back in this part of the season, not just the 100th episode but everyone we're able to bring back because of the story arc being in the Underworld," said star Jennifer Morrison, who plays Emma. "I thought it was such an inventive way to bring all these stories back that we've all loved and we've missed, so it's been really fun to see those actors and to work with everybody again."There is one person who won't be seen in the place between life and death: Robin Hood's former wife Marion."There might be [someone he's dreading]," Sean Maguire teased of his character's trip down the River Styx, but it isn't his former love. "The thing is, people go to the Underworld if they have unfinished business if they were bad, and Marion wasn't bad, so she's probably in a better place. I don't think I'm giving too much away by saying that."The most heartbreaking moment of the episode comes when Henry finally meets his adopted grandfather and namesake, Regina's father Henry. Tony Perez reprises his role in episode 100, which Pearlman calls a "very, very emotional" moment."I'm really excited for the audience to see Regina reunite with her dad. That's a big, big, big moment for Regina, especially because the last time she saw him she killed him," Lana Parrilla said of what the moment means to her character. For Jared Gilmore, who plays Henry, meeting Regina's father for the first time was "really cool.""I cried so hard filming that scene," Parrilla said, holding back tears. "Certain words that were said were things I needed to hear. It became a gift not only for Regina but for me. ... I think she was dreading to see him as well. I think she was really terrified not knowing what his reaction was going to be like, or if he even was there or where he went. She was apprehensive to find him, but I'm really proud of her for doing so because she really grows from that moment and that really closes a chapter in her life, closes that door, and allows her to move on."Regina won't be the only one confronting part of her past. Josh Dallas, who plays Charming, will again be doing double duty as Charming's evil twin James. Though James makes a dramatic, brief appearance in episode 100, Charming will get a chance to "come face-to-face with his past" later on in the season."I'm excited for these characters," said Dallas. "The Underworld is a crazy place. It's not a place you want to hang out in for too long. They're all going to confront parts of their past, so that's going to be really interesting for all these characters."

Once Upon a Time airs Sundays at 8 p.m. ET/PT on ABC. Here are 12 more teases of the upcoming season , plus what to expect with Rumplestiltskin's storyline Terri Schwartz is Entertainment Editor at IGN. Talk to her on Twitter at @Terri_Schwartz