- Antagonist: Hades.
- The only Big Bads who haven't appeared in this season are Tamara, Greg, Ingrid, Maleficent, and Ursula. Nimue appears in the first-half of the season but did not appear in the second-half.
- This is the third season in which the main characters are not in Storybrooke during the main arc. In Season 2, Emma and Snow were in the Enchanted Forest, though the other main characters were in Storybrooke. In Season 3A, almost all of the main characters were in Neverland and now in this season they are all in the Underworld.