From The Heroes of Might and Magic III wiki

Class	Air Magic
Basic cost	10
Duration	Until end of combat
Level	2nd

Reduces target enemy stack's Defense.

Level	Effect
None	Defense reduced by 3
Basic	Cost reduced by 2
Advanced	Defense reduced by 4
Expert	Defense reduced by 5

Note: This is the only stackable spell (it can be cast again at the same unit to further reduce defense).

[edit] Comment

The advantage of this spell is that effects are cumulative over the duration of the first casting, for as many times as you can cast it. The most typical use is on a Blinded stack, dropping defense down to zero before all friendly troops attack, making for an easy and quick kill of an otherwise tough opponent.

It is also impossible to dispel/cure. The effects stay until combat ends.