Retaliation
Retaliation refers to the counterattack of a stack that has been target if melee attack. Typically creature stacks have one retaliation per combat round. Griffins have two retaliations and Royal Griffins have unlimited number of retaliations. Also with the Counterstrike spell, a hero can magically increase the number of retaliations from one to two or three per combat round. Additionally, some creatures have special ability that allows no enemy retaliation. A creature will retaliate against the first creature without "no enemy retaliation" to attack it in melee (meaning, you can't use creatures with "no enemy retaliation" to prevent an enemy creature from retaliating that round).
Creatures with no enemy retaliation:
Sprite
Harpy Hag
Cerberus
Vampire
Vampire Lord
Psychic Elemental
Magic Elemental
Naga
Naga Queen
Hydra
Devil
Chaos Hydra
Arch Devil
Creatures with more than one retaliation per combat round:
Griffin has two retaliations
Royal Griffin has unlimited retaliations