Damien thought that Farah had ulterior motives whenever she slept with Tristen because when he scanned his memories, he noticed how Farah was trying to extract certain information from him.
If Tristen was alive, then he might not be experienced enough to notice it, but someone like Damien who knew about such traps could see it clearly.
He also thought that she might not be doing anything to help Kale but somebody else since if she truly loved Kale, she wouldn't be cheating on him in the first place no matter what.
So the only other person he could think of was the Gale Emperor, and since Tristen was known to have a weakness when it came to his lower member, he felt that it was quite natural for the emperor to take advantage of this weakness by using Farah to get the information he needed.
