One day after her mother dies, Twilight receives a letter from somepony claiming to be her real father. She has it confirmed that she hasnt' been living with her real dad, and humors the writer of the letter. Following the Trail of crumbs he left behind, she disobeys what she was asked, and looks deeper into the situation..
..and deeper underground.

My friend, Cole,is writing a not as good crossover about The Last Apprentice
I'm not completely sure what the series is about because I don't read books, But check his story out...
HERE: Check It Out