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The user places a hallucinatory darkness on a target's eyesight, causing them to see nothing but black; the sensation has been likened to being at the bottom of a deep hole. Because the target cannot see, they are very vulnerable to attack. Although this handicap is dangerous even to the likes of the Third Hokage, it is not insurmountable, as the Third is able to sense attacks to try to defend himself and smell his attackers in order to stage a counterattack. When the Third finally captures the user and begins removing their soul, the darkness disperses, something that Orochimaru, an onlooker, immediately notices.
In the manga and anime, this technique is called by its full name, Genjutsu: Bringer-of-Darkness Technique (幻術・黒暗行の術, Genjutsu: Kokuangyo no Jutsu, Literally meaning: Illusion Technique: Bringer-of-Darkness Technique). The databook entry for this technique drops the "Genjutsu" part.