Black Iron Einheri

Tetsujiro Soma, a samurai from the Kamakura era who survived the Mongol Invasions of Japan, suddenly finds himself in an unfamiliar place.
A mysterious girl named Hrist tells him that he is in Valhalla—a hall where the souls of fallen warriors gather.
Thrown into the battle-crazed hall of Norse mythology, Tetsujiro must battle “einherjar” from various eras in order to return to life!

(Source: MANGA MILLION)