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The Flying Thunder God Kunai (飛雷神クナイ, Hiraishin Kunai) are custom-made kunai (特注クナイ, tokuchū kunai) utilised by Minato Namikaze in conjunction with his Space–Time Ninjutsu: the Flying Thunder God Technique. They differ from a standard kunai in that they have three blades instead of one and on the handle (which is thicker than normal) is the "marker" for Minato's teleportation ability; Minato also said that it's a bit heavier than a normal kunai. The prongs on them also makes it more deadly in melee fighting.
Minato gave one of these kunai to his pupil Kakashi Hatake, as a celebratory gift for his promotion to jōnin as well as to be able to teleport to his team at any time the kunai was thrown. During the Third Shinobi World War, in the battle between Konohagakure and Iwagakure, the ninja from the former threw these kunai behind enemy lines which allowed Minato to instantly teleport to the various locations and slay the opposing shinobi, while at the same time making it difficult for the enemy to predict where he would teleport to next. He also kept several of these blades at his safe house, so he could teleport there quickly when necessary.