Get ready to say goodbye to your favorite “Once Upon a Time” characters. Season 4 of the hit ABC fairytale series is coming to a close on Sunday, and fans won’t want to miss the action packed conclusion. After the game-changing twist in episode 20, anything could happen in the new world of “Heroes and Villains.”

For those that missed the last installment, “Mother,” Maleficent (Kristin Bauer van Straten) finally reunited with her long lost daughter, Lily (Agnes Bruckner). But that wasn’t the big jaw dropped of the episode. Regina (Lana Parrilla) was getting ready to use the Author (Patrick Fischler) to write her estranged sister Zelena (Rebecca Mader) out of their lives when she had a change of heart. Instead of using the Author to get her happy ending, she decided that it was up to her to make her own happy ending. Unfortunately the Author had other plans. He used his magic ink and quill to return to Gold (Robert Carlyle), who gave him the new fairytale book, “Heroes and Villains,” to write in.

The two-hour season 4 finale will kick off with the residents of Storybrooke “scrambling” to stop Gold and the Author before they change everything. However the two are set on “turning the tables on heroes and villains alike.” The promo video for the upcoming episode shows Rumplestiltskin as a brave knight, Snow (Ginnifer Goodwin) as the Evil Queen … and Emma (Jennifer Morrison) chained up and locked in a tower.

Fortunately there may be a way to change everything back. The synopsis teases that it’ll be up to Henry (Jared S. Gilmore)to “save his family before the story’s final page is turned.” But will he be able to do so in time? The episode description for the finale, titled “Operation Mongoose,” promises a “shocking twist that will leave the residents of Storybrooke reeling.”

The two-hour Season 4 finale of “Once Upon a Time” will air on ABC on Sunday, May 10 at 8 p.m. EDT. Check out the promo video below: