Darkness.
This was the strongest feeling people got when they entered the Underdark.
The whole narrow passage was dark, the ground was a little wet because of some water, and there were many dark green mosses on the walls on both sides. Soran had night vision, so it was easy for him to see around, but the little Lolita behind him was in a bit of trouble. The dragon's vision was far better than that of humans on the surface, but it was very limited in a dark environment.
