From the introduction:
Some time ago now, a band of highly intelligent Yuan-Ti that inhabit an island some way from "civilisation" made a startling discovery. The amulet of an extra-planar power — that is to say, the device which Tanar'ri lords and princes use to maintain their vital essences — has been traced to a location somewhere in the Prime Material Plane! Possession of this device would mean having the ability to control Cassiel (whom the Yuan-Ti always refer to as 'The Beast'), a ferocious lord from the darkest and most turbulent of the outer planes. With power of this magnitude at their beck and call, the Yuan-Ti (and their human associates) would be able to dominate large areas of the Forgotten Realms. Few individuals would have even a chance of standing in their way.
From the introduction:
For 8th - 10th level characters.