The Storybrooke heroes are one step closer to defeating Hades (Greg Germann) after Henry (Jared Gilmore) found the Author’s pen on ABC's “Once Upon a Time” last week. However, the ruler of the Underworld is not one to be toyed with. He’s got other tricks up his sleeves to ensure that no one returns to the land of the living without his blessing.

The synopsis for episode 16, titled “Our Decay,” reveals that the entire gang will be continuing to search for a way out of the Underworld. But things are going to get a little more difficult when Hades forces Gold (Robert Carlyle) to transport a couple more Storybrooke residents down below. Belle (Emilie de Ravin), Zelena (Rebecca Mader) and Baby Hood will find themselves among the other lost souls, and it won’t be a happy reunion.

The episode description explains that Zelena and Regina will get into a “sibling squabble.” But that’s the least of their problems. The promo video reveals that Hades and Zelena had a secret relationship in the past!

A flashback will reveal more about their romance. It turns out that Hades initially paid a visit to Oz with the “intention of striking an alliance” — not falling in love. Certain events transpired to bring the two together. But why did they keep their relationship a secret? The heroes will be determined to find out what they're hiding.

Meanwhile, Belle will also discover a secret regarding Rumple. Will he come clean about being the Dark One again? Or will he tell her the truth about the deal he made for his second child? Whatever Belle discovers will “change her life forever.”

“Once Upon a Time” Season 5, episode 16 will air on ABC Sunday, April 3, at 8 p.m. EDT. Watch a sneak-peek video of “Our Decay” below: