The Crystal Void. The Seraph Chronicles, #2

Swashbuckling Supernatural Adventure! The year is 1810 and as Napoleon's marshals chase Wellington's expeditionary force through Spain to the lines of Torres Verdras, dashing if rather dim French Hussar, Gaston Dubois, is astonished to encounter the love of his life. But the fragrant Odette is abducted before Dubois can consummate his passion by the Marquis Da Foz, a ruthless and sadistic Portuguese nobleman.
The hot blooded Hussar is soon in deadly pursuit, but can he evade the Marquis' myriad traps and who is the mysterious ally, Major Seraph, who comes to his aid? What strange horrors lurk within the shadows of the ancient Moorish fortress where Odette is held captive? Can the heroic duo foil Da Foz's dark machinations, defeat his unnatural allies, rescue Odette and prevent the opening of the dreaded Crystal Void, before it unleashes a new reign of terror on the world? "Great story, interesting characters, lots of sword and musket action, and the potential for future stories in an underutilised setting." - Sci-fi and Fantasy Reviewer