Plot summary
The event of Emma being taken by the Darkness has the Storybrooke residents seeking a way to bring her back, and the only way to make it happen is to turn to an unlikely person: Zelena. However, the wicked witch has other plans in mind, only to be thwarted by Regina, who uses Zelena's magic to activate the Apprentice's wand, which creates a portal that brings everyone to the Enchanted Forest. They find a bewildered Emma, who is trying to track down Merlin, and along the way befriends Merida, while being guided by a dark apparition of Rumplestiltskin, who wants Emma to embrace the Darkness.
After they meet up with King Arthur in their quest in Camelot, the residents return to Storybrooke six weeks later, with no memories of what happened in Camelot, while wearing completely different attires. Emma then shows up, now a fully-fledged Dark One, and vows vengeance on her loved ones.
Credits
Adapted from Wikipedia contributors, Once Upon a Time season 5. HTML formatting and reference markers removed.