Mid-season 5 speculation, based on that one scene in Pirates of the Caribbean...

"Killian, no!" Emma cried over the wind and rain.

"Emma, it's the only way! I have to pay the price!" Killian replied, his voice shaking in fear, unscheduled tears welling up.

"It's not your price to pay," Emma sobbed, leaning her forehead against his. "It's mine because I gave in to this darkness and we can't get it out of me," she cried.

"I pulled Excalibur from the stone," he brushed her windswept hair out of her face, "I need to do this to protect what is apparently my kingdom."

"But then they'll have no king," Emma shook her head frantically.

"They'll have a queen."

"What?"

"Marry me?" Killian cupped her face with his hand.

"What?" Emma repeated, tears-eyed.

"Marry me. I love you. I don't want to die without doing something I've wanted to do since the day I met you. I know you'll save me Emma. Marry me now, and then when you find me, we can live the rest of our lives together," Killian pleaded, removing one of his rings, the one he wore on his thumb, and placed it in Emma's hand.

She nodded and looked around at her family and friends who were scattered around the dark park, fighting off monsters to buy Killian a few last moments with Emma.

"Merlin!" Emma called frantically.

"A little busy at the moment!" He replied, annoyed.

"Marry us!" Killian called.

"Oh, are you bloody serious?" He replied from across the small pond in the park.

"Please?" Emma half sobbed.

Merlin shook his head in exasperation. "Princess Emma Swan of Mistheaven, do you take King Killian Jones of Camelot to be your husband, in sickn-"

"YES."

"King Killian Jones of Camelot do you take Princess Emma Swan of Misthaven as-"

"I DO!"

"THEN KISS YOUR WIFE!" Merlin bellowed as he swung his sword and slices a fury in half.

And he did. Killian kissed Emma like he'd never kissed her before, pouring all the passion, all the love for her, every ounce of trust and hope and light that she'd brought into his life into that kiss.

And as he did, a light with incredible force shot through them, knocking them both off balance. Emma felt the darkness leave her, she felt in disappear into thin air. She looked around, but everyone was too busy fighting off the spirits who were trying to take Killian away from her.

Before Emma could even react, however, Killian was being pulled from her grasp, toward the dark red portal that led to the underworld.

"NO!" Emma screamed, but there was nothing she could do.

As he was dragged away, their eyes never parted, she could see him mouthing "I love you" over and over again until he was dragged inside the portal.

When the portal closed, Emma fell to her knees in despair.

"No," she sobbed weakly.

She felt her father's arms around her, lifting her up, embracing her as the rain stopped and the wind died down.

Her family offered their condolences, their support, but all she could think about was getting her pirate back. Her king. Her Killian. Her husband.

She would. With his ring safely around her neck, a token of heir love, she would find him and bring him back.

She did.