Twilight is now an alicorn and is looking to settle back down into her "normal" life with her friends.

Uh-huh, Right.

One rainy day, a flight practice indoors reveals a young filly sleeping
in Twilight's home. But she's another ALICORN?

What part does this new alicorn play in the battle to free Equestria from an ancient spell, and why does she look like a darker version of Twilight?

[picture is from deviantart. Only temporary art. Looking for permanent.]