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Maou to Ryuuou ni Sodaterareta Shounen wa Gakuen Seikatsu wo Musou suru You desu The Boy Trained by the Demon King and the Dragon King, Shows Absolute
A boy named Lucius is trapped in a distorted space created by the ancient hero. What he finds there is the Demon King and the Dragon King, who should have been defeated by the hero 300 years ago. Lucius befriends the Demon King and the Dragon King, and is trained by them, going from a weak child to a legendary warrior with the power of both demons and dragons.