From The Heroes of Might and Magic III wiki

Class	All
Basic cost	4
Duration	One day
Level	2nd

Skill Level	Displays	Range
Basic	Size of wandering stacks and willingness to join	greater of 3 and (Spell Power)
Advanced	Troop composition and Primary Skills of enemy heroes	greater of 3 and (Spell Power * 2)
Expert	Enemy town statistics and garrison composition	(Spell Power * 3)

[edit] Comments

Be advised, this spell does NOT nullify an enemy Hero's Disguise spell. He will still see the numbers, but not the correct creature types.

This spell can be considered one of the neutral spells. Most of the time it will collect dust but if you have Diplomacy as a skill you can better manage the collection of extra creatures. If there are 'special' dragons such as Faerie Dragons or Rust Dragons or even other 7th level units one can get quite a bargain by learning beforehand that they don't have to pay valuable resources like Sulfur for their services.