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Earlier, inside the Ancient Battlefield, Arthur fought Grand Marshal Draco alongside his golden dragon pet. He was learning the dragon riding skills and aerial combat of a Dragon Cavalier during the battle.
He had been beaten up quite a lot of times but he indeed learned many things during the process. He was also able to master the dragon riding fighting style and other things.
He felt quite good about it. He was also able to buy a lot of time for the group to recuperate outside the Ancient Battlefield, so both of his goals were pretty much achieved.
The only problem was the Power of Calamity within the grand marshal's body. It now fully consumed its host and took over the body, the mighty and honorable grand marshal was now nothing but a mindless monster of slaughter.
Arthur struggled a lot during the last phase of the Dragon's Challenge. He did everything he could just to survive it, though he was further beaten up during the entire process.