Why do Alicorns exist? What purpose do such powerful beings hold in this world?
And what happened to the others?

Royal duties used to be the only thing on newly crowned princess Twilight Sparkle's mind. When Princess Celestia summons her to discuss the responsibilities that come with the title of Princess of Magic, Twilight learns the truth about alicorns and their purpose to life in Equestria. The weight of the crown is measured in the bodies and souls of the citizens of Equestria, but that's not to say that both are needed to maintain balance and order.