Genjutsu (Illusionary Techniques) are jutsu that are generally not real, and does no real physical damage on the opponent. Those who fall for it only experience sensory illusions.

Genjutsu controls the chakra of the Target's celebral system, making them believe that they are experiencing sensations when they really aren't.

Genjutsu uses Yin Release to create a fake reality, unless combined with Yang Release via a Bloodline Limit (like Izanagi).

List of genjutsu : .http://www.narutohq.com/genjutsu.php

Genjutsu is not meant to kill victims (only in the rare case of Itachi's Tsukyomi on that his lover girl what's her name sef)

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MAKING UP GENJUTSU

I think it's much easier to make up a genjutsu than it is to make up ninjutsu. Whatever your character is doing isn't real, so however way they make the genjutsu is reliant on their creativity and skills.

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Itachi was known as the greatest genjutsu user and Kurenai was the second greatest according to canon (this does not count Madara or Shisui, and I'm not sure about Sasuke either)

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One does not need to be an Uchiha to be great at genjutsu

Other shinobi that specialised/were skilled in genjutsu :

KURENAI.
The KURAMA CLAN.
GENGETSU (late mizukage)

TAYUYA.
JIRAIYA IN SAGE MODE.

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COUNTERING GENJUTSU

–In the anime, strong smells can break genjutsu.

–if the victim is more skilled than the caster, they can break the genjutsu ("release")

–inflicting pain on oneself.

–sharingan users can detect flaws in genjutsu and break it

–ocular genjutsu can be stopped by obstructing the caster's vision.

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WHAT GENJUTSU REQUIRES

-precise chakra control

-high intelligence

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USES OF GENJUTSU

– to distract the victim

– to make a sneak attack

–to confuse the opponent

–for torturing and brainwashing

–in some smut fics, it can be used to induce an orgasm

–so play around with it.