The Armies of Daylight (Darwath #3)

Outside the great stone Keep of Renweth, the night was the province of the Dark Ones -- monstrous creatures that had swarmed out of their eldritch underground caverns to destroy humanity.
Inside, where the pitiful remnant of the once-great Realm of Darwath huddled, Chancellor Alwir and Bishop Govannin continued their bitter struggle for dominance. Against all advice, Alwir went ahead with plans to use Ingold, Rudy, and the other wizards in a hopeless, desperate invasion of the Nests of the Dark. The Bishop, obsessed with fanatical fury, sought to root out all magic by murdering the wizards. Both conspired together to seek the aid of Alketch, ancient enemy of the Realm.
In that hotbed of ambition, hatred, and fear, no one except Ingold would heed Gil's discovery of the truth that lay behind the rising of the Dark!