The Truth Will Free My Soul

Centuries after Zero Requiem, Queen Nunnally of Britannia is haunted by a spirit forcing her to call on world renowned spiritualists Sumeragi cousins Suzaku and Kaguya for help. But without knowing the spirit's identity or motive, it won't be easy. Especially when questions regarding Zero arise. Could this be the ghost of the original Zero? Or worse, the Demon Emperor himself?