The Most Fearful Tutor is the 86th episode of the Katekyo Hitman Reborn! anime.
Tsuna's training with Hibari begins. During their battle, Tsuna is trapped and enclosed by Hibari's hedgehog Box Weapon. Hibari reveals that Tsuna will die from lack of oxygen if he does not escape. Tsuna, almost reaching his limit, prepares one final attempt to burn through the hedgehog. He burns a small portion and realizes that the small portion is where the Vongola ring was. He realizes his X-Gloves flames must be equal to the Vongola ring's flames in order to break through. Tsuna then collapses from the lack of oxygen. Just then, Tsuna's Vongola ring emits images to Tsuna's head and reveals the past crimes of the Vongola.