Fog of Concealing

The village of Katzu has remained safe over the centuries because of the mages of the village.  They have developed a spell which allows them to conceal the village with a deep fog that seems to roll in from no where.  Because they are near a lake, (though not right on it), the fog rolling into the area doesn't look very suspicious, (a perception check of very hard would make some one wonder what was going on).

The spell requires at least four mages, one at each direction point around the village, but more makes the spell more powerful.  The fog will roll in to cover the village very quickly, but for those coming toward the village it appears to roll in slowly and make visibility very hard.  (An appropriate spell resistance can be used to disbelieve.)  The mages of the village make sure they have strong apprentices to learn this spell because it is so important to the safety of the village.  And if an adventuring group has proved time and again they want to help the village, the mages are likely to give the spell to a mage of the adventuring party.