Duncan was a member of the clan during its time located in the Crystalspine Reaches. One day he was just there as if he always had been and once everybody was used to him he suddenly left. He turned out to be good at fighting, so Candace and Cailan allowed him to stay. For a short while he was in a relationship with Candace, but he felt constricted even though it was an open one. It didn't last long, but wasn't a factor in why he left. Nobody knew the reason, but he did tell Candace that their relationship had nothing to do with why he had to go.
Duncan is an accidentally mysterious dragon, since he doesn't like talking and hates to do it about himself.