If foe initiates combat or foe's HP ≥ 75% at start of combat, inflicts Atk/Def-10 on foe during combat and reduces damage from foe's first attack during combat by 30%. If foe initiates combat, after combat, the closest foes within 4 spaces of target who have yet to act have their actions end immediately.
This skill can only be equipped by its original unit.
"If foe initiates combat" means if a foe triggers combat during the enemy phase. The unit being initiated upon does not need to deal damage, nor be able to counterattack.
However, if the effect is applied after combat, the unit being initiated upon must also survive it to fulfill this condition.
The stat decrease that can be applied by Nightmare is considered a temporary debuff.
If a skill such as Miracle or Everliving Domain can trigger, the damage-reduction effect will not trigger and the user will still survive with only 1 HP.
For example, if both Dodge and Gust trigger, the maximum total damage-reduction is 64% instead of 80%, since the two skills reduce the damage to (1 − 40%) × (1 − 40%) = 36% of the original damage.
The effect is triggered regardless of whether Nightmare's user performs an attack during combat. If foe initiates combat and user cannot counterattack, Nightmare's effect will still trigger.
Nightmare's user needs to survive the combat for the effect to trigger.
Nightmare can end the foe's action.
Nightmare triggers after start-of-turn effects and is unaffected by False Start.
Arcane Lúin and 
Dominance.
Miracle or 
Everliving Domain can trigger, the damage-reduction effect will not trigger and the user will still survive with only 1 HP.
Triangle Adept and the weapon-triangle advantage boost.
Share Spoils or 
Halves non-Special damage reduction.
False Start.
After Start-of-Turn Skills Trigger, Action Ends Immediately can be neutralized by effects such as 
Firm Canto Curb.
