The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 1 (manga)

The most dangerous prey…
When the most accomplished assassin on Earth meets his end just before retirement, he finds himself standing before a goddess. Her world is headed for disaster, and she has need of his particular set of skills. Armed with powerful new abilities and decades of deadly knowledge, he begins his second life as Lugh, young scion of the Tuatha Dé clan of assassins. His mission: to find and eliminate the strongest individual in this world-the hero!