The Darkest Day by Dennis L. McKiernan (Iron Tower Trilogy Series, Book 3) from Books In Motion.com

It is the time foretold, when the Dark Liege Gyphon will return from exile to subdue all creation to his vile domination. In the Iron Tower, Modru works his magics to summon his implacable lord. And across Mithgar, the warriors of the kight are massing for their final assault on the Powers of Dark. Men, Elves, Dwarves and doughty warrows, their state is desperate but their courage is high. For if they fail, there comes the unending Darkest Day. If they succeed, all will see the dawn of a New Age of legend and glory.....