Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei Suru

The greatest killer in the world has been reincarnated as the eldest son of a family of aristocratic assassins. But the goddess who granted him this status asks him for a service in return: "Kill the hero announced as the destroyer of her world"! With his knowledge, his experience from modern times, and the magic he discovers, he could well become the most invincible assassin of all time! Enough to reassure him for successfully completing this mission...