I recently finsihed a story that has elements of corruption, doll transformation and rubber transformation as well, set in the MLP universe.
A strange artifact comes into the hand of one Twilight Sparkle. It's devious nature is being revealed in front of her very eyes, and it seems she is the only hope Ponyville has. Her struggle is shown in her letters to her mentor, who for some reason sent her this odd Pearl.
The Black Pearl