The title is Lady of the Lake. I assume the Lady will be the same form-shifting siren that Prince Charming defeated in Season 1, episode 13, What Happened to Frederick.
The official description and a list of guest stars (including someone new) is after the jump (warning — spoilers):
Emma and Mary Margaret, with the aid of Mulan, Aurora and brave knight Lancelot, attempt to find a portal that will bring them back to Storybrooke. But a dark force threatens their safe return.
Henry tries to talk Jefferson into reuniting with his daughter.
Meanwhile, back in the fairytale land that was, on the eve of meeting Prince Charming’s mother, King George poisons Snow White, and the only antidote lies within the waters of the Lady of the Lake,
Barbara Hershey as Cora
Alan Dale as King George
Sarah Bolger as Aurora
Jamie Chung as Mulan
Sebastian Stan as Jefferson
Gabrielle Rose as Ruth
Sinqua Walls as Lancelot (new!)
Alissa Skovbye as Paige
The episode was written by Andrew Chambliss & Ian Goldberg and directed by Milan Cheylov.
It airs on October 14.