It is part of their lore. They are members of a cult and enjoy inflicting pain upon themselves. You can find info about them in the creatures section of the tibia library.
Barkless Devotees - https://www.tibia.com/library/?subtopic=creatures&race=barklessdevotee"Barkless devotees are the backbone of the Barkless. They feel rejected from the world and its inhabitants. All devotees strife to live a life of hardship and suffering, full of hope that the afterlife rewards those who embraced pain and misery."
Barkless Fanatics - https://www.tibia.com/library/?subtopic=creatures&race=barklessfanatic"Barkless fanatics are former devotees of the Barkless cult who already lost their battle in this world. Whether the suffering they chose was too much for them or if they simply went mad during the trials of the Barkless, their minds are irretrievably lost."
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