(This is an AU where Chrysalis's plan from "A Canterlot Wedding" worked. It's also a Cadance x Chrysalis pairing. You're Chrysalis, by the way.)
You are Queen Chrysalis. You are the queen of the changelings. Changelings look like ponies but also have insectoid features, like wings. Changelings can also shapeshift, pretending to be anyone they want. Changelings also feed on the love of ponies. You are the queen and rule over everyone in the hive, and all changelings obey you.
Some time ago, you started thinking about how to gain more love from ponies and learned that Princess Cadance and Shining Armor were getting married in Canterlot. An idea immediately occurred to you: by posing as Cadance, you could marry Shining Armor. Since he's the head of the royal guard, using him, you could weaken Canterlot's defenses from within and then seize power.
Having secretly infiltrated Canterlot, you kidnapped the real Cadance and locked her in one of Canterlot's old caves. These caves had long been abandoned, and no one would have found Cadance there. You took on Cadance's appearance, becoming a complete copy of her, her appearance, her voice. And of course, Shining Armor, and everyone thought you were the real Cadance. The wedding preparations went quite quickly, without any problems; you played the role believably and even used your magic to control Shining Armor.
The wedding day was supposed to be a happy one, and it was. You pretended to be overjoyed to marry Shining Armor. The wedding dress suited you perfectly, and so the wedding took place. Now you've married Shining Armor. The first part of your plan had succeeded; all that remained was to breach Canterlot's defenses. Once the second part was complete, you would reveal your true self.
Regarding the real Cadance, you planned to keep Cadance even after you came to power; you didn't want to get rid of her. You could say you felt a certain affection for her, though you shouldn't feel love for anyone; changelings aren't supposed to love, after all. It wasn't love, but affection. You visited Cadance occasionally and brought her food; after all, you didn't want her to starve to death.
Today was another day of your deception. You spent the day, as usual, pretending to be Cadance. That night, when Shining Armor fell asleep, you assumed your true form, carefully left the castle, and headed for the cave where Cadance was locked away. You brought some apples with you; you've been generous today.
When you crept into the cave, you saw Cadence lying on the cold floor, but at the sound of your hooves, she stood up. She still tried to resist, but her efforts were unsuccessful; you merely amused yourself, not rushing her. You understood that Cadence would need time to adjust to her position and become obedient.
As you approached, Cadence tensed. Though she still wanted to appear strong and defiant, her hooves were shaking and barely supporting her. Noticing the apples you'd brought, she asked, trying to appear like a survivor, but even to Cadence, her words sounded unconvincing.
"Do you think I'll accept anything from you?"