"Once Upon a Time" season five is not only going to Camelot, it is also going to explore darkness like it never has before. And its new season promo gives a sneak peek of just what kind of darkness.

ABC just released the season five promo for "Once Upon a Time." Just like any other promo clips, it's short but sweet. The title of the video says it all, "No one can stay good forever."

The 15-second clip opens with a dark cellar and a voice-over narrating, "She was a hero, a savior." Then Emma Swan (Jennifer Morrison) appears in her Dark Swan costume, at which point the narrator drops the line, "but no one can stay good forever."

Avid fans of the series already know that Emma was sucked by the Darkness at the end of season five, turning her into the newest Dark One. The first minute of the pilot episode shows that season six will not go far from last season's finale. It will pick up right where last season ended.

In the seaosn opener, Hook (Colin O'Donoghue), Emma's parents (Ginnifer Goodwin and Josh Dallas), Regina (Lana Parrilla), and Henry (Jared S. Gilmore) try to find where the Darkness took Emma. She is nowhere to be found and even Hook's summons are not working, which hinted that the Dark Swan might not be in Storybrooke.

This is what might lead the former inhabitants of Enchanted Forest to foray into King Arthur's world in Camelot. Casting news of Camelot's characters, Arthur himself, his wife Guinevere, and the knight Percival, have been dropped in the past few months. Merlin, who is also supposedly in Camelot, is also going to make an appearance. He, after all, is said to hold the key in defeating the Darkness once and for all.

Camelot and the Storybrooke characters converge when "Once Upon a Time" returns on ABC this Sept. 27.