Ninjutsu

Ninjutsu is the martial art, strategy, and tactics of unconventional warfare and guerrilla warfare practiced by the shinobi (ninja).
Of course the tales of Ninjas are widespread and all over popular media from video games to comic books to movies.
The invisible, silent assassins are seen everywhere.
Kind of defeats the whole idea doesn't it?

Ninjutsu was developed by people mainly from the Iga Province and Kōka, Shiga of Japan.
All through history the ninja have been seen as assassins, scouts and spies, noted for their use of stealth and deception.

Ninjutsu was developed as a collection of survivalist techniques that included gathering information, techniques of non-detection, avoidance, and misdirection, disguise, escape, concealment, archery, medicine, explosives, and poisons.

The lineage of modern day Ninjutsu schools is very cloudy.
Most modern schools are considered "Neo-Ninja" because of this.

I am in no position to argue the validity of any of the modern schools claims.
So if you are thinking of studying Ninjutsu you need to do a lot of research on your own.