(In this AU, you are the fourth member of the Dazzlings. There may be inconsistencies with canon.)
Battle of the Bands
You were an ordinary student at Canterlot High School. You were quite popular, but you didn't have the best personality. You considered yourself better than others and believed you were destined for something greater. You were never satisfied with what you had, despite your power at school, your popular friends, and much more.
A lot has changed at school in the past few months. After the Fall Formal, Sunset Shimmer, the former bully feared by the entire school, donned a crown that was actually the Elements of Harmony from another world, the magical land of Equestria. Sunset transformed into a demon and attempted to subjugate everyone.
You were one of the witnesses to Sunset's defeat by the Mane Six, led by Twilight Sparkle, who was actually an alicorn pony and the Princess of Friendship from Equestria, but after arriving in this world, she became human. After her defeat, Sunset immediately apologized to everyone, but she needed to make up for everything she had done as a demon.
Several months passed, and although Sunset had changed, she was still treated with distrust and caution. Sunset had truly become a different person, completely different from her former self. She had become friends with everyone in the Mane Six (Twilight had returned to Equestria, but she and Sunset kept in touch). You, in turn, treated Sunset neutrally, though less so than before. You and Sunset used to be friends, but not anymore. This weak, kind girl is nothing like your friend.
The school was about to host a Battle of the Bands. Three new girls, Adagio Dazzle, Aria Blaze, and Sonata Dusk, had just enrolled. The three newcomers immediately expressed their desire to participate in the competition, and they were allowed to. Their group was called the Dazzlings.
It seemed like everyone in school was enchanted by the Dazzlings (literally). You were no exception. Each of the Dazzlings had such wonderful voices, and you were immediately drawn to them. It just so happened that the Dazzlings were looking for a fourth member for their group, and they invited you.
After you got a little closer to all three Dazzlings, they told you that they were actually sirens from Equestria. You weren't so surprised anymore, considering the events of a couple of months ago. You became an equal member of the group, Adagio. Aria and Sonata treated you as if they'd known you for a long time, considering you were just like them, cunning and manipulative, albeit an ordinary person.
You felt you could get what you wanted with the Dazzlings. They let you in on their plans, which involved cultivating universal adoration and harnessing the vast amounts of negative emotions humans exude, as they needed them as sustenance. The Dazzlings promised you that you would gain immense power if you stayed with them. They also gave you the same gemstone pendant, which you could use to absorb negative emotions.
It was lunchtime. All the students were sitting in the cafeteria, or rather, everyone except you and the Dazzlings. The four of you were standing in the hallway outside the cafeteria. Adagio, as usual, explained the plan, demonstrating her leadership skills. Aria responded sarcastically and sharply, criticizing Adagio's plan. Sonata acted as usual... a little stupid and naive. Honestly, you've become a bit attached to these three.
After another stupid remark from Sonata and another caustic comment from Aria, Adagio sighed in irritation and addressed Aria, Sonata, and you:
"Okay, guys, that's enough. I'll repeat our plan, we'll go to the cafeteria, start singing, and make everyone fight among themselves. Then we'll absorb their negative emotions, and then..."
Adagio turned her gaze to Sonata and addressed her directly.
"And Sonata, please don't say anything unnecessary. You almost gave us away last time, but luckily we got away with it."