Set some time after "Devil's Due". Just assume enough time has passed for them to have the necessary angsty conversations to clear the air.
Retrieving Killian, it turns out, is the easy part. Patch him up with a little magic and he's fine. Now they have the much more difficult task of figuring out how to get back to the surface, especially now that Hades has decided to demand three of their company become permanent residents. Henry appears supremely unconcerned. He claims this is just that low moment before they win. Snow White, to no one's surprise, agrees. Rumplestiltskin occasionally suggests that everyone make themselves comfortable for a long stay but everyone is ignoring that advice.
Emma made the mistake of going with Killian and Henry to get some friendly rock trolls with unfinished business to help. With the trolls' usual excessively linear thinking, they decide that it would be best to marry the couple entirely without telling either of them, because finding out was what had derailed their first attempt to get Kristoff married to Anna, although he had realized they were right eventually and married her. It would have saved a lot of effort, according to the trolls, if they had just gotten married with the trolls tried in the first place. So this time, they just got the couple married and neglected to tell them that was what they were doing. They just sort of nudged and dragged Killian and Emma through all the right steps for a troll wedding ceremony without actually telling them what was going on.
Henry is trying very hard not to laugh. He had developed an inexplicable affinity for Disney movies during the year in New York (and then tried to inflict them on all of his family and friends in Storybrooke once he remembered who they are, with varying degrees of success, because he found watching their faces while they watched themselves hilarious.) It's not until they are standing waist deep in a hole, under a rock arch, with dry redish grass crowns and cloaks around their shoulders that he finally breaks down and starts laughing.
"What?" Emma asks.
"Don't panic, but I think you might have accidentally gotten married."
"Congratulations!" one of the trolls shouts, dumping pomegranate petals over the couple.
Emma laughs. Then stops laughing because Henry looks like he is being serious. Then she starts laughing again because what even is her life when can stand here in the Underworld, not one hundred percent sure whether her son is… well… trolling her, or whether the rock trolls married her and love of her life, Captain Hook, without her realizing. And what is weirder still is that it probably doesn't even count as the weirdest thing that has ever happened to her.
Killian raises an eyebrow. "Is that so, Lad?"
"Yup," Henry says with a grin. "They're a bit… extreme with their matchmaking."
"Of course it's so," one of the trolls says. "You're perfect for each other."
"Bit of fixer-uppers, the pair of you," another agrees.
"But you do the job wonderfully."
"Well… yes… but you don't know either us of," Emma objects. "I don't think you get an opinion."
"We're experts on love," another insists.
"And it's not like they're actually wrong," Henry points out.
"David is going to kill me. Again."
"I'm more worried about how Mom would react."
"Aye. It is about her turn to kill me," he agrees.
"Killian!" Emma objects. He flashes her a cheeky grin. "Dad likes you, you know he does. And you… or… trolls making an honest woman of me probably wouldn't bother him. Mom's cool with us, but I really don't want to find out her reaction to missing my wedding because I eloped to have a troll wedding in the Underworld."
Killian raised an eyebrow. "You seem to be taking it in stride," he points out.
"Yeah," she says, a little more seriously, "I guess I am." She smiles. "Turns out, having been forced to thoroughly considered the possibility, forever without you is a lot scarier than forever with you."
"Glad to hear that, Love."
"You seem to be taking this in stride too."
"Aye. Well, about that." He rubs a hand across the back of his neck. "I had an unfortunate chance to consider the same thing. I planned to ask, after we got out of this place of course." He gives her a slightly nervous smile and drops to one knee. "Emma Swan, will you… stay married to me?"
She smiles. "Yes. But be aware, I'm not planning on letting death do any parting."
"Aye, Love. I've noticed." He stands to his feet and takes a step closer. "Avoid anything drastically stupid and I won't expect you to."
"And we can sort out on our way back to the loft whether we count as engaged or married, given the odd circumstances."
"Aye. You deserve a proper wedding."
"What's wrong with this wedding?" one of the trolls demands.
"It was rather sudden," Emma says. "Although, to be fair, we don't have a lot of downtime between crises so I'm not sure that's a terrible thing."
"And I meant to speak with you and David first," Killian says to Henry.
"Why do you think I suggested that the wedding-happy self-proclaimed love experts would hold vital information?" Henry says. "And Grandpa would have to be crazy not to approve of you," he adds with a grin. He has read the new addition to the storybook and knows it every bit as well as the rest of the tales it contains.
"I hope he remembers that. Do you have a plan for convincing Snow White not to kill me, or you?" Killian asks.
"Ah, that. Killian, you know I love you and have gone to great lengths, not always reasonably, and made great sacrifices to make you not dead."
"Aye. I've rather noticed."
"So to keep my mother from killing you, and probably me, and then you again, I am going to make the biggest sacrifice of all. I'm going to let an honest to goodness fairytale princess plan a wedding reception once we get back to Storybrooke."
"That is quite the sacrifice for you, Love. I'm having trouble picturing you being dressed by bluebirds."
"Ugh. I'm going to hope she doesn't do that."
"There's still one obstacle to us being married, Love. We didn't say 'I do.'" He reaches up and unclasps the chain holding his ring as a necklace.
Emma smiles, her eyes lighting up the way they only do for him. "I do," she says. "I feel like I should have fancier vows or something, but I've never been good with what to say."
"Wait. Why are they getting married again?" one of the trolls asks. Henry shushes them.
"You know what I said in the past, about home being the place that when you leave you just miss it? When I thought I'd lost you, I missed you, more than I could ever miss a place. You're my home, Killian."
"And I do," Killian says. "I haven't always been a good man, you know that, but you never held my past against me, and you have always offered me a chance for a better future, to be a part of something, to be a better man than I thought possible. And I want to spend that future with you, whether in Storybrooke, or here, or any other realm. I'd even go back to Neverland with you if that's what it takes. You are my happy ending, Emma Swan, and it is an honor to be part of yours."
Emma holds out her hand and he slips the ring on her finger. Then she slips the rings off his fingers one by one, her eyes on his as she does so.
"You told me once that these rings were a reminder that all sins can be forgiven when someone loves you. And that is absolutely true, for yours and mine. But you don't have to carry them around with you because you have a better reminder now. If you ever doubt that, if you ever need the reminder, it's right here." She twines her fingers with his, so he can feel the cold metal of the ring on her finger.
"Aye, Love, I infinitely prefer this remembrance."
Henry grins. "Then by the power vested in me by nobody in particular and in these guys by… actually I have no idea… No. You know what, scratch that. I'm the Author. That comes with a tiny amount of authority and some perks, such as knowing a thing or two about happy endings." He pulls a pen, not The Quill, just a simple ballpoint, and one of the books out of his bag. "It also gives me the authority to write the best six words in a story. And weddings are basically formal declarations of happy-ever-after. So by the power vested in me as the Author I now pronounce you husband and wife. You may, and enjoy this because it's the only time your teenaged son is ever gonna say it, you may now kiss the bride."
Henry sketches the moment quickly and it transforms into an illustration, just like the rest in the book. Then he writes one line beneath it, the best six words, aware of their irony in this place, yet certain of their truth.
"And they lived happily ever after."