Mine has stuff related to who he was as a human. So like, he's a doctor so the Hippocratic Oath takes the Oath part seriously as a Changeling to the point it becomes a frailty, or he gains gold as a bane like the Dullahan. Not because he's actually one, but because he got into his durance as a result of a bad decision made in a fit of panic and stress and thus metaphorically "lost his head" because he wasn't in the right state of mind to be making that decision.