Kinjutsu are techniques that have been banned and deemed "Forbidden" from being taught or used for either their nature or the damage they can cause to the ones whom use them. they fall under one of 3 categories.
1) Techniques that can cause death to the user.
2) Techniques that violate the laws of nature.
3) Techniques that are known to cause massive collateral damage.
Many characters in the Naruto series have been known to use kinjutsu, the first of which was "Orochimaru". Others include "Tobirama, Obito, Madara and Kabuto to name a few.
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