The Mirror Pool has been a lost artifact for centuries. That is, until Pinkie Pie decided to misuse it.
Twilight Sparkle falls in to said pool while researching it. Instead of making a clone, she finds that the Mirror Pool is more than a cloning pool, but a portal to a twisted reflection of Equestria.
She manages to escape, but soon one by one her friends start to disappear, taken by what witnesses claim was her. Twilight must travel back to the mirror world, but things are not as simple as they seem when the mirror world reflects the inner selves of the ponies that fall in...
Chapters (1)
Good start, it seems you're on you way of doing what this community does best - taking small details (like the Mirror Pool) and building own theories and explanations around it. Also, I would add italics to distinguish Twilight's thoughts from the rest of the text (there's one around fifth paragraph). I'm also eager to see some dialogues (there weren't in this chapter and it's perfectly understandable). Keep it up!
Twilight wakes up and finds herself in a strange world: An Equestria, but where everypony she knew has switched genders! she has stumbled into: UNIVERSE 63!!!!!!!!