Shokei Sareta Kenja wa Lich ni Tensei Shite Shinryaku Sensou wo Hajimaru

Dwight, a Sage who helped defeat the Demon Lord that plunged the world into darkness, was wrongly accused of becoming the next Demon Lord due to his power. He and the Hero were cast into the Valley of the Dead to die. His body decayed, but he was reborn as a skeleton, driven by his lingering resentment. Seeking to challenge the world he believes wronged him, Dwight wields power exceeding that of the former Demon King. He commands a vast army of skeletons and initiates a full-scale war.