Witch Breed: Book 4 by Alan Gibbons
When Paul arrives in 17th century London, he expects to be thrown into a life or death struggle for the three gates that imprison the ancient King Lud. But the battle doesn't come. Instead, Paul roams alone, learning how to survive in a city where all the talk is of the savage civil war that rages beyond its ramparts. Somewhere underground, Lud is waiting in his crypt, preparing to rise again. War, fear and want are his tools. But Paul too has his own weapons and is gaining strength and losing inhibitions about using it. Meanwhile, beyond the city, innocent women are being killed for it is so easy to claim that they are witches. One woman - whether innocent or guilty - possesses the only power available that can help Paul in his quest.