"Our Decay" is the 16th episode of the fifth season of the ABC fantasy drama Once Upon a Time, originally airing on April 3, 2016. The episode is pivotal for its introduction of the romantic dynamic between Zelena (the Wicked Witch) and Hades, the ruler of the Underworld. Plot Summary
Zelena attempts to steal the Scarecrow’s brain to fuel a time-travel spell. She is confronted by Dorothy Gale, who has returned to Oz to protect her friends. During her quest, Zelena encounters Hades, who reveals he wants to help her.
The episode alternates between the Underworld in the present day and a flashback to Oz.