Kimetsu no Yaiba Season 1 Episode 3.
Over the next year, Tanjiro trains with Urokodaki, improving his physical form and learning the Water Style swordsmanship. Nezuko has fallen into a deep sleep that has lasted for six months, though there doesn’t appear to be anything wrong with her. One day Urokodaki declares he has taught Tanjiro all he can and leads him to a boulder. He tells him if he can slice it with his sword he will let him enter the Final Selection – only those who survive the Final Selection can become official members of the Demon Slayers Corps. Urokodaki leaves, never to teach him another lesson. Tanjiro tries for six months but cannot damage the rock, all while Nezuko continues to sleep. As he begins to give up, a young man in a fox mask, Sabito, appears and attacks him. The two fight and Tanjiro is at a severe disadvantage despite having a real sword and his opponent a wooden one. Sabito scolds him for forgetting his lessons before knocking him out. A girl with a cat mask, Makomo, is with Tanjiro when he wakes up and helps hone his techniques, like Total Concentration Breathing, which increases the wielder’s physical strength. She tells him she and Sabito were both orphans raised by Urokodaki. Sabito occasionally returns and fights him but Tanjiro loses every time. After six months of this, Sabito and Tanjiro fight again and this time Sabito has a real sword. Tanjiro’s blade finally reaches Sabito first. The adopted siblings smile and vanish. Tanjiro realizes his sword, which he thought cut through Sabito’s mask, has actually sliced the boulder in two.