From Bulbapedia, the community-driven Pokémon encyclopedia.
Night Slash is a Dark-type move introduced in Generation IV. It is a Physical move, therefore it runs off of the Attack stat of the user and Defense stat of the target.
Night Slash has a critical hit ratio of 12.5%, as opposed to the usual 6.25%.
The Japanese name つじぎり (Blade Testing) is a reference to the way in which some particularly unscrupulous samurai would test new swords in feudal Japan. This was by hiding in wait by a road (typically a crossroads) and waiting for an unsuspecting commoner (i.e. lower class, and so with far fewer rights than the samurai class) to pass by. He would then strike to kill, and in this way learn how well his new sword could cut.
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