You guys might areardy know this idk. but the Lin Kuei is a REAL ninja clan in china here is the information about it. from Wikipedia
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The Lin Kuei clan, also known as forest demons, were an ancient secretive cult that operated in northern China many hundreds of years ago. This individual clan's past and history is shrouded in mystery. However, it is believed that they resided in the deep parts of the forests, and they captured unsuspecting travelers and forced them to breed with the clan in order to grow their numbers and members. Children are usually brainwashed and were chosen at birth to work as spies, assassins, and thieves for the benefits of the clan (similar to the thugees in India). When more and more people began moving into their territories, their numbers dwindled and went their separate ways.

Probably during China's Tang Dynasty, some of the Lin Kuei members traveled to Japan to teach their secret arts and ways to the local villagers. Over the years, many of these methods were used and were incorporated into the art of ninjutsu. It is believed that some Lin Kuei stayed in Japan and made a living as mountain hermits or priests, known as the Yamabushi. They have since disappeared. Other Lin Kuei traveled to Korea and continued their teachings.

It is believed that they were monks who practiced their ways and customs. They lived secretly in caves and in forests for centuries. After the clan's decline, the last Shr-lin (grandmaster of the Lin Kuei) moved to the United States to reform the clan. The last Shr-lin died during the 1970's, and since then, a clan member known as Li Hsing, who claims to have been a friend of the last Shr-lin. Two books written by Li Hsing are China's Ninja Connection and The Combat Skills of the Lin Kuei: Heritage of the Ninja.

In Batman Begins, it is more likely that Bruce Wayne is training under the Lin Kuei rather than ninja-style, as Ras al Ghul's cult is an ancient Chinese cult that also incorporate various kung fu styles into their curriculum.

The idea of the Lin Kuei soon became most popularly known in the hit fighting video game, Mortal Kombat, in which the Lin Kuei were portrayed as a clan of Chinese ninjas. The Lin Kuei known in the game are Sub-Zero, Sektor, Cyrax, Smoke, and Frost.