Lambo vs. Large Ooyama is the third major battle of the Inheritance Ceremony Arc: it features the Lightning Vongola Guardian Lambo's future self and the Mountain Simon Ring Holder Large Ooyama fighting over their pride.
Tsuna and the others decide to camp. While they were asleep, Lambo was awakened by a rabbit in front of him and chased after it and falling into a pitfall. Tsuna and Gokudera also fell to the pitfall and found themselves in a cave surrounded by rocks. In the middle of the cave, Lambo encountered Large. Large claimed that the dome where Tsuna, Reborn, and Gokudera were trapped held the ceiling up. Lambo still oblivious to the Large's intentions tried to get him to play with him. However Large refused attacking Lambo in the process causing Lambo to cry and unleash his Dying Will Flames into his Vongola Gear Stone. The stone transformed into Vongola Lightning Helmet Version X but Lambo quickly took it off because it was too tight for him. Large then suggests playing sumo wrestlers with only one rule, the person who fall from the platforms loses. Lambo accepted Large immediately pushed him of the ledge. Large said that if only they were on the same age and if Lambo is not from Vongola, they would've been good friends. However, once the smoke clears up, Future Lambo is shown clinging to the ledge, saying that now they were now about the same age, surprising Large.
Tsuna and the others were relieved at Future Lambo's appearance, but Future Lambo told them that because of the hit, he lost his part of his past memory. However, looking around the situation, Lambo realizes that it's a one-on-one battle. Large then explained to Future Lambo that this is sumo match and only had one rule: the one who falls from the platform loses. Since the child Lambo didn’t have any pride, he allowed Large to create this rule. He also explained about his pride and revealed his Mountain Flame, acknowledging Lambo as his worthy opponent. Lambo decided to fight with his pride on the line and activated his Vongola Gear’s Cambio Forma into huge armor for Lambo, which is heavy for him he fell down. Due to Large's mountain flame that allowed him to synchronize with the earth, Lambo was knocked off by Large but able to kept his balance from falling. This made Lambo discouraged to fight again, especially after being told that the loser would be imprison at the Vindice Prison forever, he became less willing to fight and began to cry. However, when Tsuna and Reborn claimed that they regretted their decision for taking Lambo with them because he was still a kid, Lambo stated that they were wrong. In his childhood memories, he was happy to be with Tsuna and the others and wanted to grow up to truly become part of them. He then told Tsuna to always take the younger him with him from now on because he wants to be with him and the others. Finally remembering how to activate Vongola gear, Lambo charged up his flame and draws the iron sand from the battlefield towards him to form Electrico Ironhorn and attack Large with Ferro Corno Electro Shock. The attack not only destroyed Large's mountain, but also Large's armor and Flames and knocked him out of the ring, thus making Lambo the winner of the battle.
Large is imprisoned, and the second of seven "Keys" are shown.