The Armies of Daylight (Darwath #3) by Barbara Hambly

After a year of magical combat, the world is in ruins, and the few surviving wizards, including two stranded Californians, must take to the offensive Since the Dark Ones returned, the world has been laid to waste. The land’s wizards have been slaughtered, its cities destroyed, and its people scattered in terror, and few have witnessed more of the destruction than Rudy and Gil—two ordinary Californians who found their way across the Void, and took up arms in defense of a strange and magical world. She learned the ways of war, while he found within himself the powers of a great wizard. Both of them will need all their strength to survive this final challenge. Ingold, the master wizard, has devised a spell to hide the user from the deathly stare of the Dark, and he intends to use it to strike at their very heart. Finally, Rudy, Gil, and the rest of mankind’s survivors will take the offensive, bringing an end to this terrible war, for better or for worse.