Before the wyvern devoured the corpse, Arthur used a rune that allowed him to read memories. It comprised four lines working together to translate the fading electrical signals in this man's brain, telling Arthur core information like his identity and close acquaintances.
Since this man was quite ruthless, he didn't have many friends, and his name was George Larson. Then, Arthur allowed the wyvern to eat it and take revenge on the human. The monster was pleased, and it devoured him whole, leaving no evidence.
Arthur then left the pit and closed the cell, but the monster didn't mind. Instead, it slept peacefully inside the cave. Although Arthur could release it and say, 'oops, it was an accident,' he doubted that the many hunters would let it go. Furthermore, he might need an escape method from this prison.
"We are still mid-competition, Georgie Boy!" shouted one of the guards as it saw Arthur walking toward the gate. "Have you given up on our wager?"