"Yes, a letter of threat. It was from Duke Arayin. He mentioned that he knew about the research and the subjects and if I wanted this to remain a secret, I need to join the demonic cult that he is a part of along with you."
"If I agree, you all will be saved from the imperial prison before your execution by their cult and I'll remain unscathed. I'll just have to work for them in secret."
"So? Did you agree?" Berij asked.
"Of Course." The Duke replied as if the answer was obvious.
It was because, if he didn't agree, he knew that Duke Arayin would stay true to his threat and would release the news of his research and the subjects.
Then, not only would the Duke be executed, but his whole family might suffer.
For him, his family and himself always came first. If it was to save himself and his family, he'd betray the empire without a second thought.
There was no harm in joining the cult if it ensured his safety.