A classical isekai: a boy got shot and reincarnated etc.
This boy, however, found himself in the middle of conflicts between nations in the bronze age. Being a guy from another world, he used the knowledge of modern technology to assist a nation during wars.
During the journey of bloodshed, he starts to forget about his past life. Forget the emotions he used to have. Forget who he used to be. Until he became completely emotionless.