From The Heroes of Might and Magic III wiki

Class	Earth Magic
Basic cost	5
Duration	1 Round/ Spell Power
Level	1st

Reduces Melee damage done to target allied stack.

Level	Effect
None	15% reduction
Basic	Cost reduced by 1
Advanced	30% reduction
Expert	Affects all allied stacks

[edit] Comments

Don't underestimate how beneficial this spell is. It is especially useful when protecting units of lesser levels or units already with a massive amount of defense. You'll also be amazed when your hero has over 30-40 defense skill and this spell in effect, how little damage a throng of creatures will do.