Backstory: After the Movie
Instead of the Storm King's army, it was the Horde that took over Canterlot with Adora doing the Shadow Weaver's dirty work.
Adora remembered when she was growing up as a young earth filly with a dull yellow coat and sky-blue eyes, she had a clumsy walk and a best friend to help her with the world: A bipedal and heterochromic light brown cat named Catra. As a kitten, Catra stood at the same height as Adora. But as the two friends grew up, Adora grew up to have a body that was even sleeker and taller than most earth ponies. It was clear to both of them that Adora had a special destiny.
Their suspicions were confirmed when Shadow Weaver had brought back a glowing sword and gave it to Adora, causing a transformation that nopony expected.
She had turned into an even taller alicorn with bright blue eyes and a ever-flowing blonde mane in the wind, wings that were longer than Celestia's, and adorned with golden battle armor on her body and head. Shadow Weaver was overjoyed, declaring that Adora would be the Force Captain to lead the army to take over Canterlot.
Adora was proud, remembering what Shadow Weaver had said about how ponies used magic for trivial things, such as parties and friendships. And when the Horde takes over Equestria, magic will be used for knowledge and power. It'll be used for the Horde to conquer even more dimensions and territories across the universe, the possibilities were endless! Adora even dreamed of ruling different parts of Equestria with her new lover, Catra.
Or at least, so it seemed. After months of strict training, Adora appeared more than ready to wield her weapons and take down the Equestrian capitol. But even though that was successful, there was one princess that had slipped from their grasp: Princess Twilight Sparkle. The purple alicorn had taken her five friends on a journey to find new allies to take back Canterlot.
It had succeeded. Even though Adora had managed to capture Twilight, her friends came back to Canterlot with a group of parrots, a hippogriff, and another bipedal cat that looked way too similar to Catra. When Shadow Weaver managed to absorb all the magic from the four princesses with Adora's sword, she had ripped the sword out of Adora's hooves and cackled over her latest victory.
Adora felt like her heart had been ripped out of her chest, she no longer meant anything to the shadow-like being that had raised her like a mother. With her remaining earth pony strength, she fought Shadow Weaver until she tossed Adora over the balcony. Twilight saved her in the name of friendship, and in return, Adora saved her and her friends by destroying Shadow Weaver's black garnet. Twilight then used the sword to bring magic back to Equestria.
The festival commenced, with the pop star Songbird Serenade singing a beautiful song for everyone in the crowd. But Adora could hardly focus, let alone celebrate with everyone else. The ponies of Equestria faced off against the creatures of the Horde, and Adora had chosen her side when she turned against Shadow Weaver.
And Catra, her dear Catra! Where was she after all this? She couldn't have been killed, for the Equestrian ponies were too merciful. She saw stray Horde soldiers retreat from Canterlot, but Catra wouldn't have left without her right?
Her answer would quickly be answered when she saw Catra discreetly moving past the crowd to reach her, wearing none of the Horde armor that she always wore. She gently placed her paw on Adora's shoulder.
"I'm here now, let's go." Catra whispered. She immediately turned back towards the crowd to walk away when Adora placed her hoof on Catra's tail.
"Catra!" Adora whispered. "We...we need to talk."
"We can't talk when we're back with the Horde?" Catra's ears flattened.
"No, cause I won't be going back to the Horde." Adora confessed. "I fought and defeated Shadow Weaver with Twilight's help, she saved me and called me her friend."
"Ugh!" Catra spat, her nose raised in disgust. "You sound like the mushy pastel ponies that Shadow Weaver tried to keep you from becoming!"
"And how did she do that? Did she kill my family? Kidnap me?" Adora snapped. "I didn't even realize how evil the Horde was until Shadow Weaver took the princesses' magic and yanked my sword away from me."
"Adora, these ponies don't even know you like the Horde does. Like I do." Catra turned to face her, leaning her face close to Adora's muzzle. "We could've ruled part of Equestria together, and you took that away from us cause you suddenly weren't Shadow Weaver's precious daughter anymore! As if you ever were!"
Songbird's song fell on deaf ears as everything stopped in Adora's eyes. Ponies around the two started to look and inch away from them. Twilight and her friends, who were only a few feet away, galloped to Adora's side.
"You do not talk to our friend like that." Applejack stared Catra down. "Shadow Weaver has clearly hurt a lot of ponies, especially Adora."
"Adora needs to surround herself with friends that have her best interests at heart; that care about her opinions." Rarity added.
"You could be a part of this too!" Twilight offered. "Every creature under the sun and moon deserves friendship and love. Equestria could really benefit from more creatures being a part of that."
Catra glared at Twilight one moment, and looked Adora the next.
"Please Catra..." Adora whispered. "Even if this relationship doesn't work out, I want you to be my friend here. I-"
"Save it, I don't even know if you ever cared about our relationship." Catra growled. "You threw everything away when you opposed Shadow Weaver. Even when she abused me, I never turned on her. You don't know true loyalty, you don't know how to love someone no matter what. Cause if you did, this wouldn't be happening right now."
"Catra, I-" Adora stopped when Catra turned around and ran away, giving her no chance to respond. Fluttershy and Pinkie surrounded each side of her, gently nuzzling her.
"Shadow Weaver can't hurt you anymore." Fluttershy promised, rubbing her hoof against Adora's leg.
"Yeah, don't let those two meanies ruin this party for you!" Pinkie grinned.
"Thank you six for being so kind to Adora. Even though we've confiscated her sword, she needs to come with us."
The Mane 6 and Adora looked from behind and saw Princesses Celestia and Luna standing side by side. Adora gulped, stepping forward.
The two sisters teleported back to their castle, and Twilight rested her wing on Adora's back before teleporting herself and Adora back to the castle as well.