[CSteefel]

Originally Posted by Ibrîniðilpathânezel

When Legolas cries out "A Balrog!", Gandalf's response is, "Now I understand." But he never says precisely what it is he understands. That the evil thing with the orcs is a Balrog? Or that Aragorn's presentiments of danger and doom meant that he would die in this particular way? Or something else entirely?

I took this to mean that Gandalf now understood what the nature of the magical foe was that he faced behind the door at the Chamber of Marzabul. He says shortly after that:

I found myself suddenly faced by something that I have not met before

although it is true that this statement also resonates with Gandalf's early sense that some greater evil dwelt in Moria:

There are older and fouler things than Orcs in the deep places of the world.

Celeborn and Galadriel seem to have had similar thoughts:

We have long feared that under Caradhras a terror slept