Tsukuyomi

Tsukuyomi is a Mangekyō Sharingan dōjutsu and one of the most powerful genjutsu in existence. Said to represent the, it is the antipode to Amaterasu.

Overview
After making eye contact with a target, Itachi Uchiha traps them in an illusion of his design. Through his unprecedented ability to alter targets' perception of time, he uses Tsukuyomi to subject victims to days' worth of torture in a matter of seconds; examples include continual stabbing or reliving traumatic events over and over. Tsukuyomi's illusions are usually represented as an inverted grey scale; in the anime, there is usually also a red moon in a red sky. Because of the power of the Mangekyō Sharingan and the speed with which Tsukuyomi works, normal methods of countering genjutsu are ineffective against it.

Using Tsukuyomi requires a great deal of chakra, making it inadvisable for Itachi to continue fighting after using it. Like all Mangekyō Sharingan abilities, Tsukuyomi also places strain on Itachi's eyes when performed, blurring his vision. However, victims of Tsukuyomi have it far worse, as they are ordinarily rendered comatose as soon as Tsukuyomi's illusion is completed. Sharingan users can briefly resist unconsciousness, but only particularly skilled Sharingan users can actually break free of Tsukuyomi's influence. Tsunade is the only person known to be capable of healing the psychological damage caused by Tsukuyomi. In Akatsuki Hiden, Itachi theorises that victims can regain consciousness by using love and willpower to mend their spirits.

According to Itachi Shinden: Book of Dark Night, one second of time within Tsukuyomi's illusion is equivalent to 1 x 10-11 seconds of real time. In the novel, Itachi uses Tsukuyomi on Izumi Uchiha, causing her to live through decades of a happy life in the span of a few moments. When she dies of old age within the illusion, her actual body dies as well.

Trivia

 * In Japanese mythology, Tsukuyomi is the god of the Moon, said to have been born when Izanagi, after escaping from the underworld, washed his face and cleaned his right eye. As the Moon god, Tsukuyomi is associated with the passing of time, and at times is cited to be the God of Time (or at least of the calendar).
 * During the Fourth Shinobi World War, Itachi states his intention to use Tsukuyomi on Kabuto Yakushi in order to find out how to stop the Impure World Reincarnation. It's unclear if the genjutsu he ends up using is in fact Tsukuyomi, as such a form of information-gathering is usually done with Genjutsu: Sharingan.
 * In Naruto: Ultimate Ninja Storm, Itachi uses Tsukuyomi by having the red moon crush the opponent while the image of the moon warps into a copy of his Mangekyō Sharingan.