Through The Darkness: Harry Turtledove

Book III of Darkness; The World At War. As the war consuming Dervelai drags on, Algarve's resources become more and more stretched. King Mezentio undertakes war on many fronts and his main armies become sucked into the huge land mass of Unkerlant, their main enemy. As conquered races are inducted into the Algarvian army, their fighting edge softens and swift victory becomes ever more necessary, leading to the sacrifice of more and more Kaunians to allow Algarvian mages to create stronger blood magic. Meanwhile, the mad King Swemmel of Unkerlant orders the death of thousands of his own subjects in an attempt to match the magecraft of Algarve. But the most deadly magic of all is taking place on a small island where the ultimate weapon is slowly coming to fruition. Cover art by Bob Eggleton.