The Problem: Ley lines are being corrupted and mis-directed, causing unease, unrest and behavior changes in the local supernatural population. At the same time, grizzly murders are occuring, the victims being vivisected and sacrificed in some form of ritual, in seemingly random locations around the city.
Life Gets Worse: The Fae, more tightly bound to magic than mortals, are being driven crazy by the ley-line manipulation; local faeries and other magical beings go out of control and terrorize the population.
Life Gets Even Worse than That: The Summer and Winter courts both decide this is an attack by the other; the White Council decides this was caused by the Faerie courts, not causing the Faerie courts' problems, and prepares to retaliate.
The Twist: This is actually caused by a cult, wizard circle, warlock, or other power trying to bind a powerful local genius loci, and are warping the ley-lines using powerful sacrifice rituals. Can the heroes stop them before they succeed, and gain almost total power over the city?