From The Heroes of Might and Magic III wiki

Class	Earth Magic
Basic cost	12
Duration	1 Round/ Spell Power
Level	3rd

Reduces the damage target allied troop takes from earth magic spells.

Level	Effect
None	Damage reduced by 30%
Basic	Cost reduced by 3
Advanced	Damage reduced by 50%
Expert	Affects all allied stacks

[edit] Units capable of casting Protection from Earth

• Magma Elementals