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Demon Slayer season 3 picks up moments after the end of the previous season, seeing the Demon Slayers just barely alive after the fight with Upper Moon 6. After some time in the butterfly mansion, Tanjiro is recovered enough to journey to the Swordsmith village to repair his sword that was damaged in the battle.
While in the village, Tanjiro's primary goal is to repair his sword, but he is later made aware of a secret hidden somewhere in the village that can make him stronger. During his time in the village, Tanjiro plans on training to get stronger while he finds a way to get his sword fixed, and his two goals may be intertwined. As his ultimate goal is to face Muzan in battle one day, he'll need all the help he can get to become even more powerful.
**** This Article Contains Spoilers For The Swordsmith Village Arc ****
What Is The Secret Power
Hidden somewhere in the Swordsmith village is the nichirin sword of the legendary Demon Slayer Yoriichi. Yoriichi has been briefly shown throughout the show through flashbacks, having lived hundreds of years ago. Yoriichi developed the Sun Breathing technique, which he would later pass on to students who would develop it into other breathing styles, with Yoriichi being the only one to use Sun Breathing.
After his death, the sword was placed at the center of a training doll used by Demon Slayers. This training doll resembles Yoriichi and has six arms, each holding katanas, which the creator stated was necessary to match the capabilities of the person the doll was made to resemble, Yoriichi. The doll has been used for hundreds of years to train demon slayers, and Tanjiro will also do so during his time in the Swordsmith village. After many days of trying and failing, Tanjiro is eventually able to defeat the doll and destroy it, revealing the old sword within.
Why Tanjiro Needs This Weapon
Tanjiro, like Yoriichi, uses the Sun Breathing technique, which is the most powerful by far. Up until now, Tanjiro has been using the powerful Sun Breathing technique despite proper training with it, putting a heavy toll on his weapons up until this point due to the incredible and unrefined power he's trying to pass through the blades.
This sword, when used by a Sun Breathing user, has the ability to weaken a demon's quick healing ability, which is necessary to defeat Muzan as his healing is far beyond that of any other upper moon demon. This sword also changes to a red-crimson color whenever Sun Breathing techniques are used with it. This unique nichirin blade was used by the greatest demon slayer who was able to defeat Muzan, and Tanjiro will need it if he hopes to follow in Yoriichi's footsteps to take off Muzan's head.