Flagellants
Flagellantism was a 13 century and 14 century radical Christian movement. It began as a militant pilgrimage and was later condemned by the Catholic Church as heretical. The followers were noted for including public flagellation in their rituals.
Flagellation (from Latin flagellare, to whip) was not uncommon practice amongst the more fervently religious. Various pre-Christian religions, like the cult of Isis in Egypt and the Dionysian cult of Greece, practiced their own forms of flagellation.
At first, flagellation became a form of penance in the Christian church, especially in ascetic monastic orders. As well as flagellation, the rituals were built around processions, hymns, distinct gestures, uniforms, and discipline.
Initially the Catholic Church tolerated the Flagellants and individual monks and priests joined in the early movements. By the 14th century the Church was less tolerant and the rapid spread of the movement was alarming. Clement VI officially condemned them in 1349 and instructed Church leaders to suppress the Flagellants.
Modern processions of hooded Flagellants are still a feature of various Mediterranean Catholic countries, mainly in Spain, Portugal and Italy and some former colonies.
Flagellation is still common in the Philippines during Lent.
(Source: wikipedia)
For those interested in the meaning of flagellations there is an indepth article here. The revival of religious self-flagellation in lowland Christian Philippines by Nicholas H. Barker.

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To prepare himself, the flagellant either whips himself or has himself whipped by a companion.