Tsukuyomi is a Mangekyo Sharingan dojutsu, one of the strongest genjutsu. After visual contact with the target, Itachi Uchiha draws it into an illusion of his own design. Thanks to the ability to distort the perception of time, he makes the victim experience days of continuous torture in a matter of seconds: endless blows, recurring nightmares, traumas from the past. The world inside the illusion often looks like a negative with gray tones and can be supplemented by a red moon in a red sky.
Because of the power of the Mangekyo and the instant speed of the effect, ordinary ways of dispelling genjutsu are useless against Tsukuyomi. After the illusion ends, the victim usually falls into a coma. Sharingan wielders can briefly hold their consciousness, but only a very experienced Sharingan wielder is able to free themselves. Using the technique requires a huge amount of chakra and heavily strains the eyes, which is why Itachi cannot continue the fight after using it, and his vision deteriorates. The only one capable of healing the psychological damage from Tsukuyomi is Tsunade.
