A pegasus woke in the woods. He was lost, injured, and had only the vaguest of dreams for memories; nightmares of a Canterlot engulfed in shadow and flame, of an invading swarm lead by one great and terrible dark figure. As he begins a new life, the dreams continue to taunt him. Will he ever find out who he was? And if he does, can he -- and his newfound friends -- accept what he finds?
This story makes use of dramatic irony in its classic, original sense (not the modern "isn't it ironic" sense which means... well, pretty much anything). The reader can probably guess the secret behind Sky's origin and amnesia pretty much from the start, the mystery of the story is not what Sky is but rather how he'll find out and what he'll do when he does.
I also greatly appreciated that many of the characters around him were also able to figure out the obvious to some degree or another. Too often in stories like this the characters will be unrealistically oblivious to the point where you just want to scream at them, but in this case everyone acts smart and plausible.