Taitei no Ken

Yorozu Genkurou, wandering warrior, possesses an huge sword which makes him redoubtable to his enemies. While he crosses a dark forest, he suddenly sees the fall of what looks very much like a spaceship. Since, he is constantly being attacked by strange ninjas and ambitous fighters. Indeed, he will soon realize that his assailants are nothing else than aliens landed on Earth to find 3 artifacts made of Orichalcum, an alien substance with powerful mystical virtues. And one of the artifacts is the sword he wears. The hunt begins.

(Source: Manga news, translated)