Of the half-dozen types of magic in the Ethshar books, warlockry is the most mysterious -- it first appeared (in recent historical time, in the context of most of the other Ethshar novels) during the Night of Madness, when thousands of people all over the world woke up with screaming nightmares and inexplicable new powers. With that minimal background, you can probably guess what events are covered in Lawrence Watt-Evans' Night of Madness .
The book suffers a bit from prequelitis, the almost inevitable problem that when you read a book set before its postcessors (note: having determined that there's no good word to refer to the inverse of a prequel, I have decided to create one; so, for example, A Fire Upon the Deep is the postcessor to the prequel A Deepness in the Sky) you know how things end up, so some of the suspense is gone. The problem is largely ameliorated here, though; because while we know how the world turns out, we have no idea what happens to the particular characters.
Another very solid entry in the series, and another one that kept me up reading until ridiculous hours of the night. As of now, there's only one more Ethshar book written (and I don't own it yet -- it's in hardcover, and I'll buy it on my next Amazon splurge), so I'm nearly to the end of my binge. Thankfully, the series isn't over -- Watt-Evans has plans to write at least another half-dozen Ethshar novels. If that sounds bad to you, do remember that these are entirely standalone novels without even an overarching plot; there's no need to wait for the series to be finished before starting, and only normal levels of impatience accompany an intra-book wait.