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A nicely adventure to spring on the party after they have become attached to a helpful NPC that accompanied them on adventures in the past. For a variation one could also substitute a relative of one of the PC's to add a more personal touch to the adventure.
Heck, the "Old Fears Come Again" plot hook's enough for a high vote, all in of itself. Too damn often, the moral absolutism of D&D and its ilk manifests itself in the notion that if the other guys are "Evil," just about any bestial thing you do to them is okay, because, well, they're Evil, doncha know?