Units with a heat aura radiate heat into squares surrounding them. The ability value denotes the base size of the aura, which is increased by one by each level of Fire magic they have and by one for each point of Heat scales, but decreased by cold scales. Rain reduces the heat aura by 3. Heat Auras (inspector: "heat") from bless or native abilities will not work underwater, only the "uwheat" aura of the Flame Jelly will work.
The aura inflicts 2 points of armor negating heat fatigue damage three times per turn to anyone in the affected squares. Heat resistance counts double against fatigue damage. Thus, units with a resist fire(10) or greater are practically immune to the effects of heat aura.
The ability to quickly induce fatigue during a protracted fight is very helpful as enemy troops will hit less and be hit more often in addition to having the chance to be ignited.
This tactic of fatiguing the enemy works even better when fighting with a large group of heat aura units, as the amount of fatigue an enemy takes from each aura stacks. Be aware that your units will also be affected, if they are near heat auras without or low Fire Resitance (< 10). When using the aura bless, a H3 priest should be used to cast Divine Blessing otherwise the auras can fatigue out nearby unblessed units very quickly.
Taking Heat scales is a good way to leverage heat aura units, as it increases the aura spread. The bless Heat Aura on troops will have no effect in Cold3, though mages with fire paths will still spread minor fatigue.