The Spirituality skill only has one effect on the game: it's the primary determining factor for your Saving throw.
Strategy
The higher your Spirituality skill, the higher your saving throw. The higher your saving throw, the less likely you are to die to a pack of Araneas cursing you.
Characters who do not invest heavily in Spirituality will need to have more hit points, and probably blindness resistance, to be able to survive the magical spells that monsters will use against them.
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