Legacies are basic aspects of a Changeling's personality which tie them both to the Dreaming and to the Seelie and Unseelie Courts. They are the equivalent of the Archetype traits used in other Storyteller System games.
Fae souls were born of the Dreams and stories of mortals, and they feel compelled to act out their parts, becoming embodiments of popular or infamous ideals. These are the legacies, archetypal personality traits, imprints left on fae souls and personalities by the dreams of humans.
Each legacy has a Quest and a Ban. The Quest is an attitude or behaviour that, when followed, reaffirms the Changeling's legacy, renewing their will. (In game terms, they regain Willpower.) The Ban is a course of action antithetical to the legacy, and few Changelings would ever voluntarily commit such an act.
Seelie vs Unseelie
While Seelie legacies tend to be more altruistic, selfless, and law-abiding, Unseelie legacies tend to be selfish, passionate, and chaotic. It is important to note, however, that the difference is more one of motivation than of nobility or honour.
Kithain possess both a Seelie and Unseelie legacy, emphasizing the dual nature of fae souls. In the old days, before the Shattering, the fae sought balance between the Courts, with each ruling for half the year; it was seen as natural to obey both sides of one's nature, though even then most Changelings favoured one Court over the other, and thus obeyed the needs of the corresponding legacy most of the time. Since the Shattering, however, the Seelie Court has long ruled, and openly Unseelie Changelings are now looked upon with suspicion.
Some Changelings switch between legacies easily, while others may go their entire lives never changing Court. Others, either by choice (e.g., House Eiluned members can voluntarily change Courts to learn a secret), because they are cursed, or simply by natural inclination, change their dominant legacy regularly, usually according to cycles of the seasons.
In any case, such a change has significant ramifications socially for the changeling, both in their motivations and relationships with others.