Zatch Bell!

The Great Mamodo Battle (1x4)

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Zatch realizes from the battle that he was the cause of the lightning attacks. The next day, Kiyo spots Zatch trying to boast about his ability to fire lightning. They get into an argument and Zatch runs away. Zatch later meets and befriends a dog which accompanies him to Kiyo's house. At home, Kiyo is visited by the mysterious pair from before. They reveal Zatch is a Mamodo from the Mamodo World and when the book of that Mamodo is burned, the Mamodo is sent back to the Mamodo world. It is also revealed that every thousand years, a Mamodo battle occurs and the last one remaining on Earth becomes the next Mamodo King. They then demand Kiyo to surrender his book saying it would only bring trouble. Zatch, overhearing this, leaves with the dog. He runs into a mysterious man who uses a spell revealing the dog was actually a Mamodo.