Sylas' stance changed as he felt that he had observed what was happening for long enough.
While Nosphaleen and her father were talking, he was observing more than just their words—he was also paying attention to the Wills of everyone else as well.
The oddity of this dungeon was that it was impossible to see anything, but if you had a strong enough Will, you could use it as a replacement for your senses. The beauty of it was that this was a great place to train your Will, and these few Clypsians were probably better than any single expert Sylas had ever met in this regard.
But by the same token, there was a great weakness to this method as well, and that was the fact that these Clypsians had long since forgotten how to hide their emotions and their inner thoughts.
