Sorry it’s a late reply (since the story had completed), but if i’m not mistaken, Ashin talked about who would look after the castle (and maybe also look after the Duchess).
I, too, felt it was weird and didn’t make sense for an 8 year-old kid to do those kind of jobs. But when we read the things Damien could do from the POV of Ashin and Hugh (that he was thankful to be sent to school at such a young age of 6, and not to the desert; that he could kill an adult man with a sword; that he was actually a dangerous monster in a form of a small child and probably also dangerous to be placed near Lucia so Hugh told Ashin to warn Damien to be polite & respectful to Lucia), well… maybe Damien was really dependable to do the job 🤷‍♀️

Maybe it’s too late for me to reply (since this story had already completed… lol), but Hugo (the dead twin) had just died for about 5 or 6 years prior to the event of this chapter.
We can count it from the time Lucia married Hugh: she’s 18 and he’s 23 (forgot which chapter that mentioned their ages, but maybe about chapter 1 or 2). Until this chapter, they were just married for a few months (not a year yet).
The incident where the former Duke of Taran & his wife got killed by Hugo (followed by Hugo’s suicide) was when Hugo & Hugh were 18 years old (it’s said that Hugh took the Duke title soon after this incident).
Not long after Hugh became the new Duke, doctor Phillip came and brought Damien along. I forgot how old exactly Damien was at that time, but iirc he’s about 2 years old. So, I think Damien was conceived when Hugo was just about 14 or 15.
I hope my explanation is right, or someone could correct it… hahaa