Sword Art Online

The Blue-Eyed Demon (1x9)

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Kirito and Asuna flee from the boss and take a break for a snack when they meet Klein and the Furinkazan guild. The army also appears, visibly exausted from exploring the floor. They demand Kirito to give them the map he made of the floor, and despite Asuna and Klein's protests he willingly complains with their request. Kirito, Asuna and Klein keep exploring the floor, clearing the monsters there, just to learn that the army is confronting the boss by themselves despite Kirito's warnings, and to make matters worse, the teleport crystals do not work in the boss room. As the members of the army are killed one by one, they launch themselves against the boss to protect the survivors and Kirito is forced to deploy a special skill of his he used to keep in secret that allows him to fight with two weapons, one in each hand, defeating the boss with a series of quick strikes. After the fight, Kirito collapses with HP almost totally depleted, just to wake up instants later, seeing Asuna in tears, who asks him to form a party with him, to which he agrees. Some time later, Kirito and Asuna are brought before Heathcliff, commander of the Knights of Blood who challenges him to a duel. If Kirito wins, Asuna is allowed to leave the guild and join his side, but he must join the guild instead should he loses. Kirito agrees to the terms and accepts Heathcliff's challenge.