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Mass Invisibility

       Usage: cast 'mass invisibility' : cast 156 
      Effect: Makes group invisible, AC -40 
        Cost: 21 per person in group
    Duration: 12 + (level/4) hours
Casting Time: 11
      Target: None

This spells casts invisibility on all of your group members.

The classes that get the spell mass invisibility are:

illusionist: Level  20 (excellent)