The Rising.
The transformation from a pony to an alicorn, only happens when a pony has performed a feat worthy of obtaining the power of a superior to all other ponies.
It's always been passed off as a myth, a foal tale for little fillies who can't get their sleep, including the idea of an alicorn, a pony with wings, a horn and earth pony strength, too. But as war breaks out across Equestria, two sisters called Luna and Celestia find out that maybe the foal tale isn't exactly fictional after all….