Fatigue
general idea of Fatigue
"When a stack does attack, is attacked or counter attacks, that stack loses a % of its efficiency because it gets tired (Fatigue). So when it is the enemies troops turn to strike, they are less efficient if you stroke them first, and the higher your unit's initiative was, the better the odds that you managed to do so."
details
Which attacks cause fatigue? ATTACK COUNTER ADDITIONAL STRIKE EXTRA ATTACK
How much is effiency lowered? xx % Units with ability ENDURANCE: xx%