Banality is pure and simple disbelief. It is born from the rationality: the conviction that knowledge is obtained through objective and empirical observation. Magic, monsters, and faeries aren't real to a rational mind. Consensus , the weaving of individual rational belief into a consensual reality made the 20th century exceptionally banal. It stamped out individual creativity, whimsy, disempowered magic and eroded glamour. Ordinary mortals become more rational as they age. A young child has an uninhibited imagination that makes it possible for them to see changelings and chimera for what they are. But as they are robbed of their belief in all things magical and mystical, they lose that connection and forget that it was ever there. Changelings are terrified of banality not only because it destroys them as individuals, but the Endless Night, the Long Winter, the victory of banality over glamour in the Autumn World means that the Dreaming itself will die.
Autumn People are mortals who are exceptionally banal and even more lethal to the fae and chimerical creatures. Under ordinary circumstances, animate chimera avoid humans whenever possible. But an Autumn Person is leathal, because they erode the glamour of things around them, taking away the creative potential of their surroundings.
A Dauntain likewise possesses very high glamour. They are changelings who have rejected their faerie natures and become fiercely obsessed enemies of the Dreaming.