Between 1348 and 1350, the Black Death killed one in three people in Europe. In some cities, half the population. In some villages, everyone. The priests were dying alongside their congregations. The doctors had nothing. And into this catastrophe, a movement emerged.

Groups of men — sometimes hundreds, sometimes thousands — walked from town to town across Europe in white robes marked with red crosses. When they arrived at the town square, they stripped to the waist and beat themselves with leather whips tipped with metal spikes until the blood ran down their backs and spattered the walls. Three times a day. For thirty-three and a half days. In front of everyone who had gathered to watch.

They were the Flagellants. The Brothers of the Cross. They believed that their blood would wash away the collective sin of humanity and stop the plag ...