A brief description of the manga Former General Is Undead Knight:
Lambert, one of the great generals from Regios kingdom, was suspected of planning a rebellion against his lord at the end of the unification war, and as a result, he died an unnatural death.
Two hundred years later, when the era had changed and the Regios kingdom had entered its peaceful era, he was accidentally revived as a demon called Undead Knight Lambert, whose entire body was wrapped in a flashy Demon Metal Organ.
So Lambert set out on a journey to punish the wrongdoers while enjoying the peaceful sight of Regios kingdom. But… this peaceful era was basically an easy mode for Lambert, the greatest general of the chaotic era.