Elizabeth Mitchell is on her way back to ABC.

The former Lost star has booked a mysterious role on Once Upon a Time Season 4, Entertainment Weekly has confirmed.

Details are being kept heavily under wraps, but EW says Mitchell will play a character with ties to the movie Frozen, someone with a "possibly malevolent" connection to sisters Anna and Elsa.

Elizabeth Lail and Georgina Haig have already been cast in those key recurring roles, with EW adding that Mitchell will also be tied in to Storybrooke and Arendell.

Elsa, Anna and Kristoff (Scott Michael Foster) will be featured prominently on Once Upon a Time this fall, with events on the series taking place after events depicted in Frozen.

Expect everything to make a lot more sense once Season 4 premieres in September.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.