Fatigue

When a unit performs a Primary Attack or Counter attack, that unit will suffer from "Fatigue" - a term used for a reduction in Efficiency. Normally a stack starts attackin with an effiency of 100% + level of leading hero.

Use of a unit's Extra Attack does not cause fatigue to the attacker or the receiver. This is why high initiative Primary Attack units are so sought after for Fatiguing purposes.

A unit will suffer from fatigue on a Primary Attack for EACH ATTACK it makes with this attack (Additional Strike = 2 Primary Attacks, so the attacker gets fatigued twice, while the receiver who only counters once suffers from Fatigue only once).

A single attack/counter worth of fatigue for most units is a -15% reduction to efficiency (100%-15% = 85%, 85%-15% = 70%, etc. for however many hits). This % is unimpacted by the size of the attacking or countering stack. It is only affected by the unit ability Endurance, which makes the unit only suffer -10% efficiency rather than -15% for each Primary Attack or Counter.