Aeternus has seen that ominous glow several times. It happens after the body reaches its limit. The body needs to release the excess energy that is putting it in this unnatural state.
The specter told the dukes. "You better leave now if you want to live."
The dukes scrammed. The lord of chaos is not an emotional person. He allowed them to kill each other. Now he is telling them to leave now if they want to live. The seriousness of the situation cannot be more emphasized. Something extremely bad is sure to happen to them if they continue staying here. So they ran with all their might. They ran without looking back and as fast as they can go.