From The Heroes of Might and Magic III wiki

Class	Water Magic
Basic cost	24
Duration	1 round/Spell Power
Level	4th

Creates a clone of a stack, which has all of the original stacks abilities. The clone will be destroyed as soon as it takes any damage. In the late game, with hordes of troops at your disposal, this can be a much more effective "damage" spell than even Implosion if cast on a large powerful stack.

Level	Effect
None	Clones level 1-5 stacks
Basic	Cost reduced by 4
Advanced	Clones level 1-6 stacks
Expert	Clones any stack