In a clip from this Sunday's upcoming season finale, an evil Snow wants to destroy Regina for ruining her happily ever after.

Once Upon a Time's season finale will showcase a whole new side of its main characters — thanks to the Author (Patrick Fischler) setting out to write happy endings for the show's villains.

"Our finale is called 'Operation Mongoose, Part 1 and 2' — we're going to make good on what happens when the Author starts writing," Once Upon a Time co-creator Eddy Kitsis recently told THR.

"It's structured a little bit differently than we've ever done before," added Once Upon a Time co-creator Adam Horowitz. "There will be journeys into other worlds, times and places — but not quite in the way we've done it."

And in the sneak peek (above) from Sunday's finale, the true extent of how far out of whack things will be is apparent: Snow (Ginnifer Goodwin) is now the Evil Queen, and she's holding some serious resentment against Regina (Lana Parrilla) for causing her to lose her true love — who isn't Charming (Josh Dallas). (Although Charming is one of Snow's helpers in this Author-created reality.)

"I've waited so long for this moment," Snow tells Regina in the clip. "Your head is going to look lovely in my trophy room."

Ouch.

Besides the character changes, the show also is going down a different path as it sets up what season five will look like. "We certainly are setting up for what next season would be at the end of the finale this year, but we're doing it in hopefully a different way than we've done before," shared Kitsis. "It's probably a little more reminiscent, in a way, to season one, where a condition [was set out] that magic is coming, versus having a new villain of the year or a journey to a different world. It'll be in a slightly different way, but hopefully just as earthshaking or changing, than we've ever done, if not more so."

Once Upon a Time's two-hour finale airs Sunday at 8 p.m. on ABC.