This is a fictional era closer to European middle ages. Which means royalty can do whatever they want. Droit du seigneur[a] ('right of the lord') a legal right in medieval Europe, allowing feudal lords to have sexual relations with any female subject, particularly on her wedding night. Was it moral? Nope. But that is the historical and social frame of that era. So if you write a story taking place there you will take into consideration this. Royalty and church were above the law and morality. We don't have to like it, but it is what it is. You can't say "oh he is a bad man he knew that the people who he leads to his lord will be forced to sex", because simply at that era there was no consent required for the royals. Another reason they were called "dark ages" by the way, no big on human rights. That is why we evolved socially and morally.
On chap 13 so far my impression of the Duke is he is a manipulative sociopath...but he's just to ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄.
His aid is basically a magical pimp (He in the past led men including MC to the "doom room" full well knowing the ML was going to have his way with them no matter what.)
& MC looks like how most MLs in straight romance mangas do lol except instead of the domineering persona that these types have in straight romance mangas he has the typical pushover uke persona & seems to be a facecon easily forgiving the ML for sum of the fkd things hes already done or threatened... Overall...I will keep reading because hot art.(▰˘◡˘▰)