It's not easy being green — or feeling second-best.

This Sunday, "Once Upon a Time" returns to the land of Oz to explore more of Zelena's (Rebecca Mader) backstory. And it won't be pretty.

"She was really abusive, she was terrorizing munchkins, and terrorizing everybody in the Land of Oz," Mader told Entertainment Weekly.

Fans have already seen Zelena battling Dorothy. But this episode will introduce some new faces in Oz. " "We might meet some other 'Wizard of Oz' peeps," Mader teases. "We're going to learn more about why she is the way she is, and why some of the characters became who they are because of her. There's an echo between what happens on Oz, and what's currently going on, because Zelena's always been on this struggle of being incredibly pained by abandonment, and having a mean dad, and all that stuff."

In Storybrooke, Zelena has actually been helpful lately, teaming up with half-sister Regina (Lana Parrilla). Their relationship has had a few setbacks, like when she worked with the Evil Queen to bust Robin Hood out of town. But everyone is coming together to defeat the biggest foe they've ever face: the Black Fairy.

As Mader said, "It's really complicated, and the stakes are really high."

"Once Upon a Time" airs Sundays at 8 p.m. on ABC.