When the Savior becomes the "Dark Swan," the lines between good and evil blur as Emma begins to relish the intoxicating taste of absolute power. Terrified she'll succumb, Emma and Hook visit Camelot to find the one person who may be able to aid them: Merlin. Unfortunately, King Arthur proves treacherous, and when he joins forces with a vengeful Zelena, Emma and Hook's hopes are shattered. Now Emma and her entire Storybrooke family must embark on a chilling descent to the Underworld of Hades, where they'll try to rescue Hook from a fate worse than death... with "help" from Rumplestiltskin. But will any of them survive the journey?