From Introduction:
Masters of disguise that slay their victims with bluffing, hidden weapons, poisons, and cunning… Invisible demons able to trespass castle walls and the sharpest sentinels as if they didn’t even exist, they are fearsome warriors with a vast array of mystical skills. That is all the knowledge common folk living in feudal Japan had about the spies, saboteurs and murderers known as shinobi – the Ninja.
In the real world, they are the stuff from legends. In Fantasy RPG, though, we may be able to find out the awful truth – all that was said above is real! In our imagination, those mysterious mercenaries always possessed too many abilities to fit in just one model, so we decided to present, in this book, five different prestige classes that emulate various characterizations of the Ninja, each with its own focus and style, besides new feats, spells, magic items and equipment.
You, as a Player or Game Master, can choose which one fits better in your campaign, or even use all of them. Although being typical figures in oriental cultures, any world where we could find enough war, conspiracy, spying and sabotage would have room for these masters of the night.