The next episode of "Once Upon A Time" season 5 will mark the return of Zelena in the lives of the heroes. Her arrival in episode 16, titled "Our Decay," brings tons of surprises.

In a flashback, the Wicked Witch is concocting a time travel spell. One of the ingredients is a symbol of wisdom, which she thinks she will get a hold of through Scarecrow's brain.

In the "Once Upon A Time" season 5 sneak peek, Dorothy comes charging through the door before Zelena can steal the brain of the poor Scarecrow.

Also, "Once Upon A Time" season 5, episode 16 is expected to show more of Zelena's "reckless" romance with the Lord of the Underworld — something that Zelena appears to regret now.

"I've seen many powerful women in my day, but none as brilliantly wicked as you," Hades tells Zelena in the "Once Upon A Time" season 5 trailer.

It looks like whatever she had with Hades will help the heroes get out of the Underworld and beat Hades. However, with such valuable knowledge is in the hands of one of the evilest individuals they know, it remains unclear if they will ever get Zelena to help.

Zelena was able to come to the Underworld after Gold created a portal in Storybrooke, the same one that brings Belle to Hades's territory as well.

Meanwhile, "Once Upon A Time" season 5, episode 16 will also see Charming and Snow hopelessly attempt to send a message to their son Neal, who was left in Storybrooke.

"It's not a great thing that they left him behind, and they're feeling that," actor Josh Dallas, who plays Charming, told TVLine. He added that they don't want to make the same mistake as they did with Emma, which is why "they've got to try to get back" to Neal.

"Once Upon A Time" season 5, episode 16 "Our Decay" will air April 3 at 8 p.m. ET on ABC.