A ninja is much akin to a rogue in their capacity for stealth and subtlety, but where the rogue is also a master of adaptable versatility, the ninja trades these skills for a keen focus on infiltration and assassination. They eventually learn to discipline their own inner focus into strange abilities that defy ordinary mortal capacity.
Ninjas are chiefly heir to a ritualized system of self-discipline and focus originally practiced by the Kueh peoples that once ruled the northwestern corner of what later became the Ninefold Celestial Empire. The Kueh were a highly martial and sophisticated people, but their ancient empire eventually collapsed under the budding arcane might of the young Empire about twelve hundred years ago. The victors found it wisest to assimilate such an imposing foe rather than wipe them out entirely, and many modern Imperial families proudly traced at least some of their antecedents to Kueh forbears. The skills of the ninja were a cultural trophy that has since proved useful.
The bizzare talents of the ninja are not unknown in Xian. The venerable Guild of the Open Hand included some masters of the art among their number when they fled the shores of the Ninefold Celestial Empire, and these stealthy heroes did their own share of work against the goblin chieftains and witch-priestesses of their foes. Their work went unknown and unpraised, but they fought and died all the same, and their pupils passed on the lessons they were taught.
Ninjas require training to become what they are. Most in Xian are taught by teachers of the Guild of the Open Hand, though a few unaffiliated instructors exist. Rarer still are the halfling disciples of Black Ammet who are perfectly capable of learning the path that so perfectly examples Black Ammet's principles.