Darkness is a battlefield modifier that can be applied in a lot of ways. Its base effect is to decrease attack, defence and precision of units, as it becomes harder to hit someone with your sword, dodge blunt objects or aim an arrow without the ability to see them. Different ways of generating darkness will have different magnitude of penalties, ranging from -1 to -6.

Based off their abilities, units interact differently with it, some units going as far as ignoring the penalties entirely. As such, fighting in the dark can massively favor one side of a battle. Ways to generate darkness can be a significant force multiplier and research goal during the midgame.

• Units with Darkvision will suffer reduced penalties.
• Units with Spirit Sight will ignore darkness penalties.
• Units that are already Blind will not suffer additional penalties for fighting in the dark.
• Units with Dark Power will get a bonus to their stats when fighting in the dark.

• The eponymous spell Darkness, accessible after researching Alteration 6 and castable by a 4 mage, will turn any battlefield into a dark room (-6 stat penalty).
• Solar Eclipse is a less effective (-3) variant, requiring Alteration 5 and a 31 caster.

• The global enchantment Dark Skies will slightly (-1) darken battles in its owner's dominion.
• The global enchantment Utterdark will massively (-6) darken battles for everyone.
• Theft of the Sun, a national global enchantment exclusive to EA Xibalba.

• Caves and Deep Seas already come with built-in darkness (at -3).
• Fighting at night, which is 50% of assassination battles, will cause -3 stat penalties.

• Solar brilliance undoes any darkness effect. It will also cause stat penalties to units with Dark Power.
• Appropriately, getting a replacement Second Sun will undo the effects of Theft of the Sun.

Interaction with The Forbidden Light and items carrying the 'localsun' tag?