Katekyo Hitman Reborn!

Realizing that Tsuna and his friends need to become stronger before facing Byakuran, Reborn arranges for Spanner to send them, along with Yuni, back to the past. As they arrive, the day after they completed the Arcobaleno trials, they are met by the other Arcobaleno, who Reborn has called together to train Tsuna and co. Yuni uses her powers to activate Tsuna's Vongola ring, bringing forth the image of the Vongola First. He tells Tsuna and his guardians that the Vongola guardians of the first generation will determine whether Tsuna and his guardians deserve to inherit the real power of the Vongola before leaving. Reborn explains that the First's guardians will appear from the ring to test their respective successors. He then assigns Tsuna and his guardians Arcobalenos respective to their elements train them. That night, the First Generation Guardians appear from the rings of Tsuna's guardians and announce to their respective successors they will be observing their actions henceforth.