The second half of "Once Upon a Time" season 5 is returning next month. Apart from the Underworld storyline, the show will also explore the backstory of evil sisters Regina and Zelena. According to TVLine, the ABC series has cast younger versions of the characters and Regina will be played by Ava Acres, while Zelena will be played by Isabella Blake-Thomas.

Acres recurs on "My Crazy Ex-Girlfriend" as young Rebecca and she has also appeared on "Agents of S.H.I.E.L.D." Blake-Thomas's acting credits include a stint in the BBC series "The Green Balloon Club" and 2011's "Johnny English Reborn."

At age 10, Regina is described as being neglected and lonely and has no interest in her mother's royal duties. Meanwhile, 12-year-old Zelena is coming to terms with the magical ability her father despises so much.

The website reports that Barbara Hershey, who returns as Cora in the winter premiere, will likely appear as the girls' mother. Acres and Blake-Thomas will make their debut in episode 19.

Back in the midseason finale, Zelena was banished back to Oz, but she will appear in Season 5B to battle Dorothy (Teri Reeves) as the heroine returns to the magical city to save its inhabitants from Zelena. It is likely that Zelena will have revenge on her agenda as she tries to make her way back to Storybrooke once more. To beat Zelena, Dorothy will have to make new friends, unlocking a strength she never had before.

Zelena's story will tie into where the story left off in the winter finale, where Emma (Jennifer Morrison) and the rest of the Storybrooke characters are seen making their way to the Underworld to find and save Hook (Colin O'Donoghue). There, the heroes will encounter some people from their past, and also come face to face with Hades himself.

"Once Upon a Time" returns on March 6 at 8/7c on ABC.