Can someone spoil me what does ml understand now?

I need to know too

He understood why he was born as the duke's son, since the duke must have some connections with hell, he's prob a part of Satan's cult

omg seriously? I wouldn't be too surprised if that's true but how did you figure it out based on their conversation or is it more a hunch?

Mostly by their dialogue and subtle scenes.

For example: the scene of the cult sacrificing a woman; the duke always cleaning up after Adrian and letting a doctor who's treatment is sacrificing people take care of him; trying to verify if he's really Satan by asking "did you really get hurt by the holy water?", even though he kept denying the duchess' religious beliefs, and he's not a curious person, but a wary one, so he wouldn't ask such things... + Adrien saying "so that's why I was born as me" = a Pfalzgraff