sometimes after an attack in combat in HoMM VI, I see not only damage dealt/units died counts, but also Swords icon with a number appear.
What does it mean?
The 2 crossed swords icon means might damage. Certain abilities will reduce or increase the might damage of a stack of monsters.
My guess is you saw for example -1 X (crossed swords icon), meaning each creature in the stack will do 1 might damage less as long as the ability applies.
The sword icons shows that the damage done was melee damage