Nigoru Hitomi de Nani wo Negau: Highserk Senki

One day, Raizou ​​Takakura, an average salaryman, had a heart attack and his life ended that day. But, he got a second life, in a different world. Reincarnated with strong skills―isn't what happened to him, instead, he was reincarnated as a foot soldier in a small country, Highserk Empire, which is surrounded by large countries. With a new name, Walm, he was thrown into the frontline of war. Covered in blood and the stench of death, along with his comrades, he was slipping through life-or-death situations. His eyes becoming dark and muddy over time... (Source: AniList)