The Devil's Own Dice by Lazlo Ferran

Trapped in time, in a dungeon with no way out! His crippled wife is kidnapped by the witch Georgiana, now allied with the assassin sect Concilium Putus Visum, and transported back to feudal 13th Century France. He is only one man against the vicious and murderous forces of Hell unleashed once again in the form of the shape-shifting Biblical Serpents. Suddenly trapped in an escape-proof dungeon he must somehow escape, and train to be a knight so he can enlist the help of a corrupt count. Along the way he must overcome his sense of revulsion about his own supernatural power. But what will he find in the mysterious Maze Tower? If you love Dan Brown, Anne Rice or Hilary Mantel you will adore this rich, twisting, writhing erotic masterpiece.