This is the tale of the horrifying days Princess Cadence spent in the depths of the Canterlot caves. Just before what was supposed to be her wedding, Princess Cadence is kidnapped, trapped and replaced. She spends what feels to her like eternity in these caves, unable to escape with no hope of being rescued.
Will her day ever be as perfect as she imagined?
This is essentially "A Canterlot Wedding" told from Princess Cadence's (the real one) perspective.
Cover art: http://panda-iz-me.deviantart.com/art/Trapped-by-a-fake-299716815