The House of Sidéral is the "hereditary" ruling family of the nation of Näktergal. Legend (or propaganda) has it that the family are the offspring of a god (or other supernatural being), and, as such, are only half-human. The fact that all of them are apparently at least 100 years old - but appear to be in their 20's or 30's - would imply that the stories are more than just myth (or that the family has access to extremely advanced medical technology).