The Pathfinder Bestiary, and the long line of monstrous encyclopedia that came before it, do a great job of attacking the problem of providing game statistics and mechanical information for a very wide variety of creatures. There are a lot of strange creatures from fantasy novels and movies, from ancient mythology, and, as time went on, from the twisted minds of game designers and even GMs, for that matter. Anyone today who tried to go about creating an extensive bestiary of all the different cool and interesting fantasy monsters could fill three or four volumes with three hundred monsters each, and still have a long way left to go, with fans crying out for various "forgotten" monsters to get some attention. That kind of breadth doesn't leave a whole lot of room for depth, and that's where Exotic Encounters comes in.
There are all kinds of reasons why you might not want to use a monster stat-block straight from the core rules. For one thing, if your players have a habit of browsing through such books, or are long-time veterans who know the basic ins and outs of most fantasy staples, you may need an unusual stat-block just to throw them a curve ball and teach them that they can't count on their out of character knowledge to take all the mystery out of the game. Alternatively, many GMs can recall a time when they wanted to make use of a certain monster in their game, only to find that it was a few CRs too high or low for what they had in mind.
Exotic Encounters takes a single, iconic monster, and creates three new variants on that theme. These variants aren't simple tweaks, and are more than simply advancing or removing Hit Dice, though that occurs as well. Each of these three variations on the monster's theme has a specific, flavorful goal in mind, and a role to play, and their statistics entries are gently massaged in order to make them fit those roles. Further, each of these variants comes with brand new specially-crafted special abilities, which are unique to Exotic Encounters and not found anywhere else.
This particular installment of Exotic Encounters focuses on the vargouille. These strange bodiless fiends date back to the first edition Monster Manual II, and are one of the stranger creatures of the lower planes, feared far and wide for their deadly kiss, which provides a very unforgiving "save or die" experience at a very low level, a terrible fate that is only compounded by the fact that the transformation is detailed in particularly macabre fashion as the character's hair falls out and their facial features slowly twist over the course of one or more days. The first vargouille in this book, the singing vargouille (CR 3) can also cause such a transformation, though in this case, it is done by a haunting lullaby, instead. Singing vargouilles can also unleash powerful and damaging sonic shrieks, and their bite inflicts a curse of silence. These terrors pale before the bloodsucking vargouille (CR 5) however, which does not kiss its victims but instead attaches to them and drains their blood, transforming their victims into wights, whose heads separate from their bodies and become vargouilles when they are destroyed. Finally, the giant vargouille (CR 7) is a vargouille of titanic proportions, with a propensity for swooping down and swallowing human prey whole in a single gulp, and either allowing them to be digested by powerful acid that inflicts difficult-to-heal wounds, or simply spitting them back out again in the air, leaving the victim to fall to his doom.