Dehydrate

This awful spell causes water to evaporate rapidly from the target creature's body. The creature loses 1 hit point per round as its body fluids seep to the surface of the skin and are absorbed by the spell. This continues until either the creature is dead or the spellcaster is disrupted. Drinking a canteen of water stops all damage for 4 rounds and restores 1d4 points of damage taken from this spell. The spell not only steals its creature's water, it transfers it to the caster as well. The priest doesn't have to drink again for a number of days equal to 10% of the stolen hit points. If 20 hit points were stolen, the priest does not have to drink for 2 days.

If the creature survives, drinking good water returns all but 1d6 of the lost hit points, and healing spells work normally.