"Another Alicorn!? Thanks, Hasbro."

(You) are an Alicorn! One that Twilight, and everypony else for that matter, is completely unaware of.

The name is [Princess (you)], but you don't have to actually be a "princess", just a pony with wings + horn.

After reading about a mysterious chamber beneath the Castle of the Two Sisters, our favourite lore hunter ventures deep underground into a mysterious room. Locked from the outside with magic chains, whatever is in there is serious business. And, what might that be? You, of course! Are you an evil Alicorn banished for crimes against Ponykind, a top-secret, super cool Alicorn kept hidden, or something weirder? I dunno, you tell me.