Units with a chill aura radiate cold into squares surrounding them. The ability value denotes the base size of the aura, which is increased by one by each level of water magic they have and by one per point of Cold in a province, but decreased by heat. The aura inflicts ((5+ Enemy cold vulnerability)/3 roundup) + DRN points of armor negating fatigue cold damage 3 times for each point of its post-Temp scales value per turn to anyone in the affected squares. Cold resistance counts double against this damage because it is fatigue damage. Units with a resist cold(3) or greater are highly resistant to the effects of Chill Aura.
The ability to quickly induce fatigue penalties during a protracted fight is very helpful. the enemy also has a higher chance to become Frozen if they stand near them for too long.
This tactic of fatiguing the enemy works even better when fighting with a large group of chill aura units, as the amount of damage an enemy takes from each aura stacks.
Be aware of your units will also be affected if they are in a chill aura without Cold Resistance.
Going Cold is a greater way to leverage your scales if going in with chill aura units, As it increases how much chill a unit takes before getting into combat and effects a greater radius of them.
Note that Having High Heat in a province of the battle is a way to reduce their effect.
Chill auras do become a wasted ability if your enemy has or can give his units Cold Resistance.