It is traditional for the Wood Elves not to launch into battle until one of their champions has loosed a shot at the heart of the enemy leader. Seldom is this arrow fatal, but fatality is not the intent. Rather, it is a goad to the enemy’s pride and a reminder of their frail mortality.
Once deployment is complete, but before the roll-off to determine which player takes the first turn, if the General of your opponent’s army is within 36" of one or more models in your army that has this special rule, one of those models may fire the Arrow of Kurnous. If the Arrow of Kurnous is fired, the General of your opponent’s army immediately suffers a single Strength 3 hit, with no armour or Regeneration saves permitted (Ward saves can be attempted as normal).
However, if the Arrow of Kurnous is fired, your opponent adds +1 to their roll when rolling off to determine who takes the first turn.