When Ashen Hallow is active, this aura shows its remaining duration, taking into account whether you're wearing the Radiant Embers legendary. This aura also tracks cooldown reduction on Ashen Hallow and assumes that if the cooldown is reduced, you've left Ashen Hallow and it is no longer active. You can also set this aura to perform certain actions (e.g., change the icon color) when you're wearing Radiant Embers and the remaining duration is lower than a particular value. (See "Conditions".)
When Ashen Hallow is not active, this aura can be configured either to show the cooldown/availability or to hide itself. (See "Custom Options.")