Jim Butcher's The Dresden Files: Down Town #5

Harry and company — along with an unwelcome ally — have tracked the Golem through the dank labyrinth of Undertown, and now find themselves in the very heart of madness and evil! Now, they must face the unstoppable creature… but even if they can defeat it, can they defeat its dark master?

Characters: Harry Dresden, Johnny Marcone, Molly Carpenter