Tsukuyomi

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.

Tsukuyomi requires eye contact to be performed. When executed, the jutsu traps the target in an illusion that is completely controlled by the user. When Itachi uses it, he is able to make the genjutsu seem to last for days when in fact it only lasts a few moments. This allows him to torture the target for what seems like days on end, causing mental trauma that will render them unable to fight for an extensive period of time. Only Tsunade has been shown capable of curing this psychological damage.

In the anime during Part I, Itachi's use of Tsukuyomi typically featured a red moon and a cloudy atmosphere that casted a red light throughout the illusion, giving the background a blood-red appearance while the victim within the illusion is in a grayscale, inverted color.

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. According to Kakashi, Tsukuyomi can only be broken by a Sharingan user that shares the same blood with the user. However this is not the case, as Danzō broke through Sasuke's Tsukuyomi.

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.

Itachi prowess with the technique was recently reiterated by Danzō, who claims that Itachi's mastery has allowed him to alter the perception of time with ease, and denounced Sasuke's use of genjutsu as inferior.

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.