A Certain Magical Index, Vol. 22 (light novel) by Kazuma Kamachi
Fiamma of the Right, last remaining member of the shadowy Roman Orthodox organization, God's Right Seat, has finally set his long-awaited plans into motion. The Star of Bethlehem: a giant fortress rises into the wartorn Russian skies. Fiamma calls for a purge of the unworthy while three young men continue to fight, carrying various causes in their hearts as they make a last stand against impending disaster.