The fallen / Eric Van Lustbader.

The End of Days has arrived and the legions of Hell are upon us...In a hidden cave in the mountains of Lebanon, a man makes a fateful discovery. He has been chosen to bring what has been forbidden for thousands of years out of the darkness and into the light: the Testament of Lucifer. In Istanbul, Braverman 'Bravo' Shaw, member of a secret Franciscan splinter sect, has been warned by his ageless confessor, Fra Leoni, of a battle most ancient: the war between Good and Evil, waged to a standstill since time immemorial. Now an unfathomable danger has arisen: Lucifer's advance guard, the Fallen. All of humankind is at risk of being enslaved by the forces of evil. Bravo, Fra Leoni, and Bravo's blind sister, Emma, are the first and last line of defense against the unleashed chaos. All roads lead to the Book of Deathly Things: the testament of Lucifer. But if Bravo and Emma become privy to its dreadful secrets, they may well forfeit far more than just their lives.