Truth Theory: Kingdom of Valkyria (真説・ヴァルキュリアの王国 Shinsetsu・Varukyuria no ōkoku) is the eighteenth novel of the Strike the Blood series.
Kojou Akatsuki invited by Princess La Folia to the Kingdom of Ardigia is told that Natsune Kanase will accompany him and decides to accept the invitation. In Ardigia, a ceremony to commemorate the conclusion of a peace treaty with the war kingdom is scheduled, and while the royal palace of Ardigia is attacked by a mysterious monster, a large-scale terrorist plan involving the war kingdom begins to move. Kojou were inevitably involved in the vortex.
- Chapter One: The Letter of Invitation
- Chapter Two: At The Night Palace
- Chapter Three: Princess's Desire
- Chapter Four: A Night In Verterace
- Chapter Five: Traitor In The Sky
- Chapter Six: Hanabi
This volume was based on the two episode OVA, Strike the Blood: Kingdom of the Valkyria.