At the time that this blue light that lit up the sky in Nord became prominent, the sun had completely set.
The Players had already readied themselves to face the next onslaught of the battle and some of them thought the blue light was some type of weird attack that the Dark Beings were preparing to unleash on the battlefield.
Some people were more intuitive than just thinking that the blue light was a type of attack. The first thing that they noticed from the blue light is that it didn't look controlled, and then also, they were sure that even during the day time they couldn't see any structure that stretched that high into the clouds.
After thinking and adding one and two, some of the Players concluded that the Dark Beings were building something crazy within the city and the need to infiltrate the city became ever so necessary and evident to them now.