Noroi no Maken de Koufuka Training!? Shirarecha Ikenai Kamen no Boukensha

The third son of a rural noble family, Tris, dedicated himself to training with the goal of becoming an adventurer. One day, he was gifted a magic sword of unknown abilities. Until the age of 15, unaware it was a cursed blade, he spent his days tirelessly swinging the sword. Without realizing it, the boy had been undergoing ultra-high-intensity training due to the sword’s power. Before he knew it, he had acquired hero-level strength!?

(Source: Kadokawa, translated)