The entire Taiping Rebellion.

A war started by a Chinese peasant who dreamed (and believed) he was Jesus’ younger brother. Although poor, the first thing he did was have a giant demon slaying sword forged. Took over a city. Asked the British why they wouldn’t pay him tribute as the new head of their faith. Engaged in total war with the Qing. Applied pseduo-communist policies like abolishing private property. Separated women and men from ever interacting, and sent the women to the front lines.

Over 20 million people died, with some estimates as high as 40 million. It was the fourth deadliest conflict in human history.