Monstrum Nox. Books of Ys, #2

When Adol Christin arrives in the historical city of Balduq, famous for its ancient fortress serving as a high security prison, he doesn't expect to get arrested at the city gate by the prison warden himself. Nor does he expect that an escape attempt would throw him into an encounter with a mysterious woman, whose magic bullets can turn a person into a Monstrum-a supernatural warrior, trapped in an eternal battle with monsters born from people's nightmares.
To break his curse, Adol must embark on a perilous journey into the past and unravel the centuries of dark secrets lurking in the mysterious depths of the Balduq prison. But nothing can possibly prepare him for the ultimate enemy waiting for him at the end of the path. Thrown into a deadly battle, Adol must stake his sanity and his very survival against the ancient terror that had plagued the land since the beginning of time.
MONSTRUM NOX, a bestselling game by Nihon Falcom Co., is part of the award-winning Ys series, featuring Adol Christin, a legendary swordsman and adventurer. This novel follows the story and characters of the game and is officially licensed through an agreement between Dragonwell Publishing, Dragonwell, LLC, and Nihon Falcom Corporation.