Nigoru Hitomi de Nani wo Negau - Highserk Senki

An average salaryman named Raizou Takakura died one day after suffering a heart attack. However, he was given a second chance at life in a new universe. Instead of being resurrected with powerful abilities, he was reincarnated as a foot soldier in the little nation known as the Highserk Empire. An Empire that was badly located and is bordered by many larger and more powerful countries. He was thrust onto the battlefield under the new name of Walm. Walm and his companions were slithering through the several life-or-death scenarios they encountered, covered in blood and the stink of death. Walm and his companions encountered an adventuring group known as the "Heroes," transferees with superhuman strength, who were capable of using strong magic. Walm began to progressively hone his fighting skills during a bloody conflict with the "Heroes" that he would never have discovered had he remained in peace. He advances gradually, albeit his eyes grow dirty and dark with time.