Tsukuyomi

Named after the Shinto Moon god, Tsukuyomi is one of the most powerful genjutsu in existence. It is unique to the Uchiha clan and only those with the Mangekyō Sharingan can perform it. It is said to represent the, the antipode to Amaterasu, a ninjutsu of similar power and fellow Mangekyō Sharingan dōjutsu, said to represent the.

Tsukuyomi requires eye contact to be performed. When executed, the jutsu traps the target in an illusionary world that is completely controlled by the user. While it only takes a few seconds to complete in the real world, the user can make it seem as if days have passed for the target. Depending on the user's whim, they can either torture their target for what seems like days on end, or make them relive a traumatic event over and over. When the jutsu ends the resultant psychological trauma will render the target unable to fight for an extensive period of time, and possibly suffer a complete mental breakdown. An example of this is Kakashi Hatake, who had to be hospitalized for quite some time after Itachi performed the Tsukuyomi on him.

A common feature of illusions created by Tsukuyomi is a red moon that casts a red light throughout the illusion, giving the background a blood-red appearance while the figures within the illusion are in a grayscale, inverted color. This feature however was not present when Itachi used Tsukuyomi on Sasuke Uchiha in Part II.

Such a powerful technique is not without its disadvantages, however. Due to the complexity of the illusion and the quickness with which it is executed, an enormous amount of chakra is necessary but had little to no effect on the stress that is put on the eyes, leaving Itachi perfectly fine after performing it despite his loss of chakra. Though Kakashi says that Tsukuyomi cannot be escaped, Sasuke has been able to break out of Tsukuyomi by using his own Sharingan.

Tsukuyomi-induced comas are however, curable, as shown when Tsunade quickly and easily countered its aftereffects affecting Kakashi and Sasuke, though admittedly, no one but she was able to, hinting the mental damage it causes is considerably high, and the victims had to stay in bed for a little while longer, looking rather disoriented after being healed.

Madara Uchiha's Moon's Eye Plan involves acquiring the Ten-Tailed Beast's power and body in the moon, which allows him to cast an throughout the entire world, effectively letting him conquer it.

Influence
Pulling from the ancient Japanese myth of Amaterasu, Tsukuyomi, and Susanoo, where when Izanagi washed his face after escaping the underworld his right eye became Tsukuyomi, the Moon god.