The illness must've been from their family making sure they keep their pure blood you know ,the incest
And they are related to the past king as well they were his uncle right?
And seme and his twin already had it planned from the start although it was never mentioned ,their whole plan ( I think there are too many plot holes the details were all skipped) uke just got involved because the seme became obsessed with him
To be fair, the entire series has been from Ilic's point of view and he never understood the duke's motivations or feelings. When you think about it, a lot of stuff in the main series didn't make sense without the context of the extras. The duke revealed a lot of sensitive information to Ilic, such as stuff about his sister, nephew, and the politics of their family.
Even when Ilic literally punched him in the face, the duke still forgave him. We also don't see the duke interested in other lovers and the duke puts a lot of effort into keeping Ilic by his side. we can infer from this that the duke was actually serious about their relationship even before he realized that he loved Ilic.

Now that comments are back I have to say: what the fuck is this story? I originally liked it, toxic or not, but now they've made the duke/king? Out to be not only twice as evil seeming as he was before but they tossed in mental illness... For what? To make him look sympathetic? The haitus didn't kill this story, the weird side story did that on its own.