[3][4] Existing fictional characters introduced to the series during the season include the main antagonists for the two volumes, King Arthur, Nimue and Hades, alongside Guinevere, Merlin, Merida, Percival, Hercules, Megara, Zeus, Poole, Jekyll and Hyde.

The season also saw the return of numerous deceased characters from previous seasons, including the Blind Witch, Cora Mills / the Queen of Hearts, Peter Pan, Cruella de Vil, Baelfire / Neal Cassidy, Milah, Prince Henry, Liam Jones, Prince James, Gaston, Claude, and Stealthy.

The battle to destroy the darkness leads to the death of Hook, prompting Emma and the heroes to travel to the Underworld, where Hades, as well as most of the deceased villains, have plans of their own to return to the land of the living.

Back in Camelot, Arthur is desperate for aid in his quest to unite Excalibur, leading him and Zelena to Dunbroch to find an enchanted helm.

Regina is mourning Robin Hood when Mr. Gold tethers all of Storybrooke's magic to the Olympian Crystal fragment in a scheme to awaken Belle.

He then learns that he has inadvertently trapped the rest of his family in the Land of Untold Stories, where Dr. Jekyll succeeds in separating himself from Mr. Hyde in the meantime, wishing to join the heroes in Storybrooke.

After a pep talk from Emma, Henry gives a speech to New York citizens about believing in magic, and asks them to make wishes in the park's fountain to help bring back his family.

[77] On June 9, 2015, it was announced that Rebecca Mader and Sean Maguire had been promoted to series regulars for the fifth season, portraying their characters Zelena / Wicked Witch of the West and Robin Hood, respectively,[10][78] while it was later revealed that Michael Socha would not be returning as a series regular as Will Scarlet / Knave of Hearts although further appearances as a recurring or guest star were hinted.

[79] During Comic-Con 2015, Ginnifer Goodwin, Lana Parrilla, Colin O'Donoghue and Robert Carlyle were confirmed to return as Snow White / Mary Margaret Blanchard, Evil Queen / Regina Mills, Captain Killian "Hook" Jones and Rumplestiltskin / Mr. Gold respectively,[5][16] while Morrison's character was confirmed to take on the moniker "Dark Swan".

[15] On July 1, 2015, it was announced that Liam Garrigan would play King Arthur during the first half of the season,[11] while Gabe Khouth and Lee Arenberg confirmed via social media that they would be returning as Sneezy / Tom Clark and as Grumpy / Dreamy / Leroy[13] respectively.

[12] Adam Horowitz announced on Twitter that Amy Manson had been cast in the role of the Scottish archer Princess Merida from Brave.

[5][16] Addressing rumours surrounding the character's origins for the show, Kitsis and Horowitz stated, "we are sticking to the canon of the movie Brave in terms of who her parents are and where she came from, but post-movie is where our Once Upon a Time spin comes into play".

[81] The premiere episode of the season, "The Dark Swan", saw the return of David-Paul Grove as Doc,[9] Faustine Di Bauda as Sleepy,[9] Jeffrey Kaiser as Dopey,[9] Michael Coleman as Happy,[9] Mig Macario as Bashful,[9] Raphael Alejandro as Roland,[9] Timothy Webber as the Apprentice[9] and Ingrid Torrance as the Severe Nurse[9] who is revealed during the episode to be the show's version of Nurse Ratched from Ken Kesey's One Flew Over the Cuckoo's Nest,[82] and introduced Lee Majdoub as Sir Kay[9] and Mckenna Grace as young Emma.

On October 14, it was announced that Adam Croasdell was cast as Hook's father, who would appear in a flashback before the midseason finale,[83] and was later revealed to be called Brennan.

[35] During October and November 2015, multiple recurring characters from previous seasons were confirmed to be returning for the 100th episode, "Souls of the Departed", including Barbara Hershey as Cora,[22] Robbie Kay as Peter Pan,[25] Giancarlo Esposito as Magic Mirror / Sidney Glass,[27] and Emma Caulfield as the Blind Witch, who was last seen during the first season episode "True North".

[86] On November 16, it was announced that Bailee Madison would be returning in the same episode to reprise her role as a young Snow White, joined by newcomers Jonathan Whitesell and Kacey Rohl, who would be portraying Hercules and Megara, respectively.

[87] On December 4, it was announced that Rachel Shelley would be reprising her role as Milah, Rumplestiltskin's ex-wife and Hook's ex-lover, for at least one episode in the second half of the season.