Take a look at the following screenshot. Full size here.
as you can see, only one of these enemy armies has an "aura of light" surrounding it. This faction doesn't have any settlements left, so maybe that's got something to do with it?
Arqade is a question and answer site for passionate videogamers on all platforms. It only takes a minute to sign up.Sign up to join this community
The screenshot shows 3 different auras, here is what each are and what they mean:
The character to the right only has one aura, which is red: the red aura only appears on characters that have the red skull above it, which represents attrition damage, see this answer.
The character to the left has three different auras, one of which is barely visible (notice the red aura at the unit's feet). The white aura replacing the red one, which is surrounded by Rays of God only appears on characters which you are currently inspecting by left clicking on it.
The last visible aura in the screenshot is the white circle of light at the unit's feet. It only appears on units that are currently in the "Forced March!" stance, allowing them to move twice the usual distance on the overworld map at the cost of not being able to attack (they can still defend, however), as well as penalties in field of view and morale during combat.
Fleet have the same aura for their own version of "Forced March!", which is called "Double Time!", as well as a similar aura for their "Patrol" stance, which is a white circle of vapor floating above the fleet.