As night fell over Baltimare, things were quiet... except for a local favorite eatery, where the patrons inside were all snarking and sniping at each other, talking loudly, rudely, and basically arguing over the pettiest of reasons, all the while none of them noticed a pale green mist forming beneath them as it slithered along the floor towards a table in the corner.

At this table sat three young mares, wearing hooded cloaks as they softly and subtly vocalized before they each took a long and deep breath, breathing in the green vapor as it gave only a minor relief before they stopped singing and lowered their hoods.

"That was barely worth the effort, Adagio. I am tired of fast food, I need a meal!"
"Well what do you expect, Aria?! Our powers aren't what they once were, so now we can only gain so much negative energy."
"URGHH, if it weren't for those unicorns we'd be ruling Equestria by now!"
"Well, at least they're gone... for all the good that does us."
"For realsies?"

Adagio and Aria looked to Sonata, who was pointing out the window towards a billboard, "Because... isn't that one of those unicorns, but with wings?"

The three of them pressed their snoots up against the window and gaped at the billboard on top of the building across the street, displaying the seven alicorn princes and princesses, with Celestia at the top, Sombra and Luna to her left, Cadance and Shining Armor to her right ,and below there were the images of Princess Twilight Sparkle and Prince Midnight Blaze.
"I don't believe this... that is him!!" Adagio was incensed that one of the two ponies who'd interfered with her and her sisters was not only still alive after a thousand years, "And he's an alicorn when we've been turned into pitiful ponies?!"

"Alright, that does it. All the votes are in," Aria said in deadpan as she slouched in her seat, "Life is now officially unfair."

"Well, I'm the one who had to pay for our table!" Sonata whined, "You guys are unfair!"

"And being stuck in this land in these inferior forms, especially with you two," Adagio seethed through gritted teeth, "isn't making this situation anymore... bearable."

"Then why don't you girls do something about it?"

!!!

Adagio, Aria, and Sonata looked towards the pony who'd just spoken to them, seeing her, a unicorn garbed in a wine red cloak of velvet as she relaxed at her own table while enjoying an iced tea before she looked in their direction, her face hidden in the shadow of her hood, "Unless you're content to live the rest of your lives on scraps..."

"Who... are you?" Adagio narrowed her eyes and the stranger came over to join them at their booth table, making herself comfortable next to Adagio.

"The name's Sunset Shimmer," the unicorn lowered her hood, revealing her brilliant amber coat and amaranth mane with pale gold highlights, her greenish cyan eyes smug as she said, "I'm here on behalf of my mentor to extend... an invitation."

"What kind of invitation?" asked Sonata, leaning forward in interest, only for Aria to pull her back.
"Just go to sleep, Sonata," she grumbled.

"Well, I couldn't help but overhear you saying... that particular alicorn on that billboard?" Sunset pointed to it, specifically at the sapphire blue alicorn stallion, "He used to be a unicorn, went on some time-travel thing, messed with you three in the past? Well, nowadays he's an alicorn prince, and for the record, his name is Midnight Blaze."

"And you're telling us this... why?" Adagio raised a suspicious brow.

"Because," Sunset sat up straight, giving them a menacing look, "like the three of you, Midnight has interfered with my ambitions, and the destiny that should have been mine became his and his precious wife's, that purple princess, Twilight Sparkle!

"In other words, ladies... we share a common interest - Getting revenge upon the pony who ruined our goals and left us with nothing but our hate and our bitterness."

The three sisters shared a look and none of them could deny the truth of what Sunset was saying. So, Adagio gave the unicorn a shrewd look, "We're listening..."

Sunset smirked in satisfaction, Catrina is gonna love this!

Adagio, Aria, and Sonata were originally sent through a time portal that had no exit, so they were lost in time for over a thousand years. Thanks to a hiccup in time, they were finally released, but some magical anomaly transformed them from Sirens into ponies:

Sonata into a Unicorn (because she's the most incompetent), Aria into a Pegasus (because she's rebellious), and Adagio into an Earth Pony (because she hates it)

They still have a semblance of their Siren magic, but their pendants were destroyed; they basically have a flicker left and they have to maintain it by using their dwindling power to absorb negative energy, but it's a losing battle. Like a candle that's going to burn through all of its wax