Once Upon a Time” is doubling up on the action on Nov. 15, airing back-to-back episodes that promise to shed some light on Emma’s (Jennifer Morrison) terrifying transformation into the Dark Swan and reintroduce some familiar faces, including Mulan (Jamie Chung), Ruby (Meghan Ory) and Dr. Whale (David Anders).

Variety spoke with creators Adam Horowitz and Edward Kitsis ahead of the two-hour installment, in which the duo teased the conclusion of the Dark Swan arc, the impending 100th episode and what we might see in the second half of the season.

What can we expect from the back-to-back episodes? Were they conceived to be one story?

Horowitz: When we were asked to do the additional episode, it wasn’t part of the initial plan to make it a two-hour story, but what we did do is, we found what we hope is a really engaging story for both of the episodes — and there is a connective tissue between them, but they tell two separate stories. But one pushes right into the other one and kind of takes a left turn a little bit, but we think it’s a lot of fun.

Kitsis: The first hour we are really going to get a lot of the answers to the mysteries and questions we’ve posed in the first half of the year and the second hour’s going to be a really great Merida (Amy Manson), Mulan and Ruby hour.

What was the appeal of that combination of characters?

Kitsis: I think you’re going to see through the flashback that two of them have a preexisting friendship, and then… we’ve been dying to get Meghan back and she was gone all of last year – she was on another show, she was out of the country, so we felt like we can’t just have her show up at Granny’s and pretend like nothing happened. There was talk of, when the ice wall came down she was on the other side as a wolf and goes “no one was looking for me?!” but we’re gonna explain where she’s been and why she hasn’t been on the show for the last year. So for us it was a fun way to give some room to people in the second hour that we haven’t seen for a while and really let them have their own thing to do.

In the last episode, we saw Emma hesitate to reforge Excalibur after remembering Merlin’s (Elliot Knight) childhood warning to her – how much will we see her struggling between those dark and light impulses?

Kitsis: We’re gonna see a lot of it, and I think this weekend is going to explain a lot of why she did what she’s done. A lot of people are saying “what is Emma doing, what is her master plan, what does she want?” and I think we’re gonna get an answer to a lot of that on Sunday night.

So you’re not holding too much back for the midseason finale?

Kitsis: No, in fact we feel like this is just the start of where we’re about to take off, and the remaining three nights until our winter finale are going to be really non-stop… This weekend we’re gonna get a lot of answers, and then the following two weeks, all those questions will be answered.

You’ve now positioned Mr. Gold (Robert Carlyle) as a hero to counterbalance Emma’s darkness – how will we see that newfound heroism manifest itself?

Kitsis: Gold as a hero is really going to be tested significantly, and I think that he’s going to be put on the spot and we’re gonna see how brave he really is.

How much control do the previous Dark Ones have over Emma – is it that she’s always in charge and they’re like the devils on her shoulder, giving her a nudge, or will it be more of an internal struggle?

Kitsis: I think we’re gonna see both. We’ve seen Emma do a really  good job of fighting the darkness and we also saw her rip out the heart of a 13-year-old and dump her son, so it’ll be a bit of both.

There seemed to be shades of “Aladdin” in last week’s episode, with Merlin like the reluctant Genie, forced to do a master’s bidding even when he doesn’t agree with it. What kind of threat does he pose now that he’s under Arthur’s (Liam Garrigan) control?

Kitsis: I think the threat he poses is, Arthur has figured out a great loophole to get one of the most powerful sorcerers on their side, but for Merlin, this is everything he’s fought against, so he is compelled to do a lot of things, but at the same time, you’re gonna see him have to fight his own darkness. That showdown is absolutely going to be front and center for the next few episodes.

Will he be sticking around for a while?

Horowitz: We’ll see some more of Merlin, and we love Elliot.

Kitsis: He’s fantastic. I can tell you that he plays a really big part in the curse and what’s happening with Emma, and we’re going to see him being really central to the conclusion of the Dark Swan arc.

You’ve now introduced the Greek god Prometheus into the world of “Once Upon a Time,” via his Forge and his Spark, and you’ve previously described the second half of the season with the word “hell.” You’ve also cast Greg Germann as a character with god-like powers — does that mean we might be seeing characters from “Hercules” in the back half of the season?

Horowitz: I would say that right now, we’re just focusing on the end run of the first half of the season…

Kitsis: …But that what the second half of the season will be, will be revealed in the finale, and I think the clues are absolutely there for people to put together.

The episode description for Sunday’s first installment, “Birth,” says Hook (Colin O’Donoghue) goes to new and desperate lengths to get answers from the Dark Swan — care to elaborate?

Kitsis: You’re gonna get a lot of Hook and Emma in the next batch of episodes, and he’s reaching his boiling point. He wants answers and he’s going to get them Sunday night, but like all things, be careful what you wish for. If you ask a question, you may not like the answer.

Said episode description also reveals that “Zelena’s (Rebecca Mader) pregnancy mysteriously accelerates,” which sounds like bad news…

Horowitz: With Zelena, everything is a little bit scary.

Kitsis: Everything is a bit of a wild card [with Zelena] … It’s out there that the pregnancy has been accelerated, but the question is, did she accelerate it or did someone else?

Horowitz: And if someone else did, why?

We’ve already heard that a number of dead or otherwise absent characters are returning for the 100th episode, including Peter Pan (Robbie Kay), Cora (Barbara Hershey), Sidney Glass (Giancarlo Esposito), and the Blind Witch (Emma Caulfield). Can you reveal how we’ll see them, if not why? Perhaps in flashbacks, visions or another realm?

Kitsis: How about all three?

Since it’s shaping up to be quite a reunion, have you been trying to get Jamie Dornan back?

Kitsis: Jamie Dornan is the number one person people ask about and unfortunately, we cannot make it work, his schedule is too busy. We will not be seeing him… You find a star and then other people grab them and then don’t share them. We love Jamie and we wish him nothing but the best, and though we’re sorry not to see his face in the 100th, I think he’ll be there in spirit as Emma wears his shoelaces on her wrist.

Anything else you can reveal about the 100th episode at this early stage?

Kitsis: The hundredth episode for us — and the second half of the season — what we’re really excited about is, we feel like we’re going to be honoring the show by really giving you that season one feeling all over again.

Once Upon a Time” airs Sundays at 8 p.m. on ABC.