Tsukuyomi

According to Zetsu, 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 technique traps the target in an illusion that is completely controlled by the user. Itachi's prowess with the technique has allowed him to alter the perception of time with ease within the genjutsu to make it seem to last for days when in fact it only lasts a few seconds. 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. After Itachi used it on Kakashi in Part I, Kisame was surprised that Kakashi was still alive. 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 casting a red light throughout the illusion with a cloudy atmosphere, giving the background a blood-red appearance while the victim within the illusion is in a grey scale, inverted colour.

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 and an added amount of stress is placed on the left eye, leaving Itachi's vision to be more darkened. Before using it on Kakashi in Part I, Itachi stated that Tsukuyomi can only be broken by a Sharingan user that shares the same blood as him; as demonstrated when Sasuke overcame Itachi's Tsukuyomi in their battle. Sasuke can also perform Tsukuyomi with his right eye, though his illusion is considered inferior to Itachi's Tsukuyomi since it does not alter the opponent's perception of time indicating he has yet to master it.

Madara Uchiha's Eye of the Moon Plan involves acquiring the Ten-Tails' 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.