From The Heroes of Might and Magic III wiki

Class	Air Magic
Basic cost	12
Duration	1 round/Spell Power
Level	Third

Target allied troop takes less damage from ranged attacks.

Level	Effect
None	Damage reduced by 25%
Basic	Cost reduced by 3
Advanced	Damage reduced by 50%
Expert	Affects all allied troops

Note: While supposed to protect from Arrow Towers as well, it actually increases the damage they do. Use with caution.