Twilight finds the mirror-portal behind Princess Celestia's throne after being left behind at the wedding rehearsal. But instead of Canterlot High, it sends herself, Rainbow Dash and Spike to Terra before the Unification Wars.
"With the vast, endless ocean of the Warp and the infinite web of the Multiverse a definitive factor; all possibilities are possible, all realities are real." - (I do crossovers a lot)