If you want to bring the Clavis Inferno into your OSR game, here's the effect:
Upon reading the book cover to cover you have 1d6 nights of nightmares about devils cavorting in an infernal garden. On the final night the devil Vepar scratches you with the infernal mark. This shows up on your body in the morning, burning and oozing. The mark will cause you great pain should you enter a holy site to a non demonic God, causing 1d6 damage for evey day you remain tgere. Should you enter a demonic place of worship regain 1d6 HP for every day you remain.