It was a dark and stormy night. I know that's a cliche, but hey, don't blame me, just telling it like it is. The Great and Powerful Trixie, having saved up a bag of bits working on some pathetic rock farm was running through the woods, headed to a town where she'd heard a shop was selling a rare and dangerous magical object. She hoped the rumors were true. It had been almost a year since Twilight Sparkle had humiliated her, costing her her job, her home, and worst of all, her pride. She'd show her. She'd show everypony in Ponyville that they shouldn't have hurt her.
She suddenly heard the familiar hum of magic coming from the woods, a soft pink glow coming from between the trees. She went to investigate, and was shocked to see Twilight, in the middle of the woods, using her magic to spin rocks around in the air. What luck! If she could defeat Twilight here, she wouldn't need the amulet from that shop! And if she lost again, there was nopony around to see and embarrass her! This was the best plan ever. She decided to approach her.
"Behold! It is I! The Great-" She was cut off by a scream. She looked at her rival just in time to see her horn stop glowing. There was a crash as the rocks fell to the ground. Except for one, which found it's way directly onto Twilight's head with a sickening crack. Trixie could only stare in shock as the purple unicorn crumbled to the ground. "H-hey, are you ok?" She approached the mare to make sure she wasn't hurt. As she got close, she felt herself step in something wet. Igniting her own horn, she looked down screamed as she saw she'd stepped in a trail of blood that was quickly spreading from under Twilight's mane. *OMIGOSHOMIGOSHOMIGOSH! I DIDN'T MEAN- I- I- NO! I just wanted to humiliate you, not... oh, no, this is bad! This is very bad! The Great and Power Trixie can't go to jail! It was an accident! Please, get up!"
She looked around for help, but quickly realized they were alone. Even if they weren't, she couldn't let anypony find her with the bleeding body of the mare who'd ruined her life. They'd never believe it was an accident. And that's when she heard the stream. Using her magic to carry the limp unicorn to the edge, she went over her options and came to a conclusion. She would have to dispose of the evidence. It was an accident, but what's done is done, there was no going back now. She wraped the body in her black cloak and lowered it into the river and watched as the water carried her away.
"What has Trixie done?" She wondered aloud. Sitting down, feeling sick to her stomach, she pondered what to do next. And that's when she realized. Twilight had friends who would notice she was missing, but nobody would notice if TRIXIE was gone. In order to hid her terrible crime, she would have to BE Twilight. Focusing all her magical abilities, she cast a spell that changed her. Her beautiful blue coat darkened to purple, her mane reshaping and recoloring until it resembled the fallen mare's. looking at her reflection in the water, illuminated by her magic, she was satisfied that she could fool others. She began walking towards Ponyville, doing her best to remember the names of Twilight's friends, when it hit her. She didn't have Twilight's memories. She couldn't fool anypony! Unless...
Taking a deep breath, she ran head first into a tree. Wincing in pain, she felt her head. The bump was already forming. Perhaps she could claim memory loss. Yes, that's perfect, she'll say she bumped her head and her memory got messed up. That should work, right? It's not like she'd have to do this for long. She could just stage some sort of 'accident' to make it look like Twilight vaporized herself or something. Twilight's all about magic, so that's plausible, right? She goofed up some spell and exploded. Maybe even twice. Perfect.
And with that, Trixie continued to walk to Ponyville, ready to start living a lie to cover her crime.