This story is a sequel to Days of the Dead
Luster Dawn, Twilight's most faithful student, is a studious young mare who knows far too much about the afterlife & keeping secrets. Recently, she's uncovered a new secret, a rather dark one that she can't talk about, as speaking of such a secret puts her own life at risk.
Sombra, former tyrannical king of the Crystal Empire, is the King of the Dead, dedicated to saving his subjects from the curse of the afterlife. Under his rule, they are not allowed to reveal themselves to the living, though he has made an exception for Luster. Unfortunately, this has put both her and others in danger.
Twilight Sparkle, Princess of Equestria, is unaware of these secrets, the dead within her castle, or her student's meddling in their afterlives. Surely, these secrets can't last forever? Especially when it begins to threaten the ones she cares about?
I didn't leave room for text on the cover art, oops