Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru

The world's greatest assassin spends his final moments betrayed by the organization he served all his life. Instead of disappearing, he meets a goddess who offers reincarnation into a world of magic with one condition: kill the Hero before that chosen savior destroys the world. He accepts and is reborn as Lugh Tuatha Dé, heir to an aristocratic family whose public duty hides generations of assassination work. Lugh combines modern knowledge, the techniques of his new family, and magic chosen specifically for his mission, training from childhood with far more freedom than he ever had before. Yet assassinating a prophesied hero is not a simple contract, and Lugh intends to understand his target before deciding exactly how the mission should end.