Creature Abilities
Source Alien Core pg. 208
The following creature abilities are listed here because they are shared by many creatures or are highly complex. The statistics for individual creatures might alter the traits, the number of actions, or other rules of these abilities. Anything noted in a specific creature's stat block overrides the general rules for the ability below. In these abilities, “monster” is used for the creature that has the ability, to differentiate it from any other creatures the ability might affect.
Aura
Source Alien Core pg. 208
A monster's aura automatically affects everything within a specified emanation around that monster. The monster doesn't need to spend actions on the aura; rather, the aura's effects are applied at specific times, such as when a creature ends its turn within the aura or when creatures enter the aura. If an aura does nothing but deal damage, its entry lists only the radius, damage, and saving throw. Such auras deal this damage to a creature when the creature enters the aura and when a creature starts its turn in the aura. A creature can take damage from the aura only once per round. The GM might determine that a monster's aura doesn't affect its own allies. For example, a creature might be immune to a monster's frightful presence if they have been around each other for a long time.