Spotting the enemy at night is difficult. Not only is it dark, but the shifting shadows make identification of enemy troops uncertain. Warriors on both sides become confused and disoriented and their performance becomes unpredictable, some failing to fire at nearby enemies whilst others continue to blaze away confidently at long range.
Before a unit fires you must check if they can see the enemy. Roll a 2D6 and multiply the result by 3, giving a number between 6 and 36. This is the number of inches the unit can see. If any enemy are visible then they may be fired at as normal. Note you should only roll for each unit once, not for each model or weapon in a unit. Guess range weapons may fire as normal, but the distance they scatter is doubled if the target is beyond visual range.