For a long amount of time, the changelings have been stuff of myth until the attack on Canterlot. Since then, Equestria has been on high alert for changelings that may be hidden among their ranks. But, before the attack on Canterlot, one pony will discover a secret he has not known in his year in Ponyville. Former Royal Guard, Terminal, sets out to Canterlot to stop Chrysalis' plan, but he is too late and gets attack, and eventually passes out. When he awakes, he has no memory of what happened, and finds himself in the presence of the changelings. As he goes on, he writes his story to keep as a memoir to himself and others.

Dark and Tragedy tags maybe?

-Radoran