Harry Turtledove 
Through the Darkness [EPUB ebook] 
Book Three of The Darkness Series

As the war which consumes Dervelai drags on, Algarve’s reserves become more and more stretched. King Mezentio undertakes war on many fronts, and his main armies become sucked in to the huge landmass 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 Algervian mages to create ever stronger blood sorcery. 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 coming slowly to fruition.