The self-mortification may serve as public penance, exemplary self-discipline, self purification or an act of partial self sacrifice. In more sinister faiths the deity worshipped may even take pleasure in the flagellant's pain.
Speculatively - and in a more magical than theological manner - the mortification may also be part of the letting required by blood magic, a way of attracting spirits or a way of tilting karma in your favour as per the "law" of equivalent exchange.
The most popular image of flagellants is probly that of medieval Romanists flogging themselves in an attempt to drive off the Black Death, but this is semi-historical at best2 and flagellants have been a part of most religions at one time or another - Shi'a Islam is another faith in which self-flagellation is suprisingly popular. Other notable practitioners include some forms of buddhist, various shamanic traditions and some Hindu Sadhus.
Game and Story Use
- This would be an … interesting … character concept - blood magician or theurge (or even a fully blown cleric) who has to self-mortify to power his spells.
- It would also make a severe - but not unrealistic - discipline of faith for a character. Depending on the extent of the self mortification it may intefere with all sorts of things - like the ability to wear armour or a backpack - not to mention the physical damage and likely constant smell of blood…