Sylas already knew that what happened to the Duchess was a plot of Thessa, a woman he thought he had killed long ago. It was hard for Thessa to stoke Nosphaleen's Envy any more than she already had, but this was certainly a method.
It didn't take a genius to realize how Nosphaleen would react to finding out that not only had Sylas taken another woman, but said woman was also pregnant with his child. After everything she had gone through with Cassarae, it was only natural.
Listening to the Duchess' story, Sylas didn't learn anything he didn't already know. Although the Duchess didn't seem to have any idea who Thessa was, Sylas could just tell by the undercurrents of the story alone that someone was stirring things in the background.
They had been planning out all of this for a very long while, and Sylas couldn't help but wonder if he was responsible for the start of Legacy as it existed today…
