Circe can manipulate nightmare matter, but it manifests into the other persons greatest fear/nightmare, which can either prove incredibly powerful or completely irrational, or just plain spiders. lots of spiders.
When Gala was little, AJ and Rarity helped her make a quilt, letting her embroider a pattern into it, and then hence gaining her a cutie mark - this quilt is still very important to her, she can't sleep well without it
