Mighty Gorgons

Please help me understand how the "grayened" Gorgons and their "breath" ability work. I read in the game manual that each powerful Gorgon has a 10% chance to "breathe" on a target. So, what does "each" Gorgon mean? Does it mean a 10% chance for the entire stack, or for each Gorgon in the stack? For example, if there are 50 Gorgons in a stack, does each of them have a 10% chance to "breathe"? And if, say, five out of 50 Gorgons trigger the breath ability, how will that look? Will it happen five times in a row? Does that mean that if there are 10 Gorgons in a stack, the probability is 100%? I've never been able to understand how the chance of triggering is calculated. Please help me understand, I would be very grateful!