Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru

The world's greatest assassin had a lifelong loyalty to his organization, but it turned against him and he was killed. In his frustration, he was granted a second chance by a goddess. She wanted him to be reincarnated in a world of swords and magic, so he could save it from destruction by slaying its hero.

Lugh Tuatha Dé was born as the son of a noble family of assassins in the Alvan Kingdom. Under his father's teachings, he learned new assassination techniques that didn't involve the cold-blooded killing of his previous life. He also made many allies and acquaintances. Despite this, Lugh knows that he must become perfect to defeat the hero and save the world.