A My Little Pony: Friendship Is Magic Dark Fic by NodoubtbuodoN.
It takes place the day after the main six defeat Discord. Celestia tasks Twilight with returning the statue to Canterlot's gardens. While she is there, however, Twilight remembers what Discord told her about Celestia's habit of turning ponies to stone. She scoffs at the idea; surely her teacher would not be that cruel. On a whim she turns to a different statue and casts a cure spell on it. The result sets off a dangerous chain of events, and Celestia's secrets may not be the worst of them.
Can be read over here
.
