the basic concept of this AU is that major protagonist roles are swapped with characters which are antagonists originally (and vice versa), whether just episode antagonists or outright villains!
 
Princess Adagio, otherwise known as the Princess of Morningsong, has ruled over Equestria for as long as anypony can remember–but as her student Tempest discovers during one Summer Sun Celebration, she didn’t always rule it alone. 1000 years ago, Adagio’s younger sister, Princess Aria, let the bitterness in her heart take hold and was transformed into a monster: Crescendo Blaze.
 
With the help of the Elements of Harmony, the Princess of Lullabies was restored to her uncorrupted form and reunited with her sister.