The Crystal Void by John Houlihan  

The year is 1810 and as Napoleon's Grand Armee chases Wellington's expeditionary force through Portugal to the lines of Torres Verdras, the dashing if rather dim French Hussar Gaston d'Bois is astonished to encounter the love of his life.

But the fragrant Odette is soon swept away, abducted, before d'Bois can consummate his passion by the Marquis Da Foz, a ruthless and sadistic Portuguese noblemen. The hot blooded Hussar is soon in deadly pursuit, but can d'Bois save both his true love's virtue and his own life and who, truly, is the mysterious British ally Major Seraph who comes to his aid?

What strange horrors lurk within the shadows of Da Foz's ancient Moorish fortress and can the heroic duo foil Da Foz's dark machinations, defeat his unnatural underwater allies, rescue the delightful Odette and ultimately prevent the opening of the dreaded Crystal Void?