Dazzlings

The Dazzlings are the main antagonists of My Little Pony Equestria Girls: Rainbow Rocks. They are a musical band group consisting of Adagio Dazzle, Aria Blaze and Sonata Dusk. They are defeated by Sunset Shimmer.

Background
The three sirens are first seen in a city cafe, where they get negative energy from the customers there, and they witness the explosion that defeated Sunset Shimmer in the prequel and they decided to harvest all the negative energy of all the Human World. The arrive in Canterlot High and Sunset Shimmer gives them a tour around the school, but she receives a bad vibe from them, as Adagio stops Sunset from touching her crystal necklace. Then, they begin to sing in the cafeteria, making the students fight with each other. Twilight informs about they, said that they are in fact sirens that were banished to the Human World, just when they first attempt to defeat they, the plan had foiled. The sirens them decided to when they get energy from the students, they get from the Rainbooms.

They then are seen in the show stage seeing the students fight and getting energy. When they seen Sunset Shimmer again, they played with her emotions and said that "no one is going to remember her when they win". When the Rainbooms pass to the finals, Trixie refuses to accept this and the Dazzlings then trick Trixie into eliminating the Rainbooms from the competition. However, thanks to Spike, the Rainbooms managed to escape.

When they were about to get the energy of the students and rule the Human World, the Rainbooms appeared and battled them. When they managed to defeat the Rainbooms, Sunset Shimmer steps in to help they out and the students begins to sing along with them. The Dazzlings appear shocked, when they see a giant rainbow alicorn that defeated them and destroyed their crystal necklaces, resulting in them losing their ability to sing forever. The students booed at the defeating Dazzlings' horrible off-key final attempt to sing and threw vegetables at them, forcing them to flee the stage in humiliation.

Trivia

 * Flashbacks reveal their original siren forms in Equestria where that of Hippocampus.
 * A lot of fan art erroneously depicts their alternative forms as mermaids, being the most common form for sirens.
 * They have, by far, more villains songs than any other character in MLP.
 * They are the second main villains with a villain song, first being Queen Chrysalis. Discord was reformed at the time of his song.
 * When humans are kept immortal for hundreds of years as the Dazzlings were (being banished over a thousand years ago and presumably kept alive by their penants), and then lose their source of immortality, typically they age to either a skeleton or dust (examples being Mother Gothel, Edgar Volgud, Dorian Gray, Queen Beryl, or Adam Monroe). This does not immediately happen to the Dazzlings when their pendants are destroyed, as they have time to sing off-key and run off the stage; however, due to their absence from future films, it is possible that their accelerated ageing may have been slower and they eventually aged into dust after running off the screen.
 * It is believed that their Fiendship is Magic issue of the comics depict them as arriving on the human world in the present day, meaning Starswirl would have had to send them through time into the future (effectively replicating King Sombra's curse on the Crystal Empire). However, nothing has confirmed this aside from appearing in a modern day city, and it is unknown whether the history of the human world is the same as in real life. Either way, the comics are sometimes inconsistent with the show.
 * In the comics, it is hinted that the sirens are redeemable. In the show, however, they are one of the five main villains not to reform, other four being Queen Chrysalis, King Sombra, Lord Tirek, and Principal Cinch. However, in the comics, Sombra reformed, while Nightmare Moon, depicted as a separate entity from Princess Luna, perished in his place.
 * The Dazzlings are the only villains in the Equestria Girls series whose transformations do not come with actual tails, but rather hair extensions like the Rainbooms (as opposed to Demon Sunset and Midnight Sparkle). Their siren forms do not count because they are astral projections, although these were their original forms before becoming human.
 * It is believed that their father was a powerful but intensely most wealthy creature that'd Equestria even ever seen and somehow have something far more sinister than taking revenge on Star-Swirl the Bearded, whose exiled his concurrent daughters to the human world. It's about bringing out something far more sinister than the Rainbooms, which was unknown.