@Ohgodpleaseno "I'm kind of confused as to why he has to assassinate her in the first place?"
It's a little vague, which I assume is because they're trying to squeeze novel chapters into manga chapters, but the implication to me seems to be that Dia's father took the hit out on himself and his daughter to end the fighting. Their retainers and peasants are standing up on their own to try and prevent Dia from being married off to the traitors, and it's created an incident that is spiraling out of control, so he must have decided that the cleanest way to end the pointless struggle that's just going to get good people killed is to arrange for his and his daughter's deaths. That way, the traitors won't get what they want, his daughter won't have to suffer being forced into a loveless marriage with some traitor to their country that she's never met, and the most amount of their loyal servants and citizens will survive, since with the two of them dead, there will be no further purpose in fighting.
It's actually a pretty good plot, all said, and it's made me excited again for what's coming next. Some people have been really critical of this series, both because parts of it are clearly compressed, and also because it's already started to deviate from the novel (apparently). But I've never read the novel, and I think it's done a pretty excellent job so far. I do wish they would slow down a bit and do some more explaining, however, even if that drew out certain scenes and episodes into multiple chapters. Because in some places, like in this chapter, I think the additional exposition would really help. Then again, strictly speaking, we haven't even really reached the main story yet, where Lugh meets up with the hero that he needs to help defeat the Demon Lord and then kill immediately afterwards before he becomes a tyrant. So from that perspective, they could just be treating all of this as a very long prologue to set up the real story, which would make sense. I have no idea how far the novel is or if it's even reached that point yet.
Obviously Lugh intents to "kill" her without actually killing her. But it remains to be seen exactly how he intends to do that and make it convincing enough to pass. Perhaps he had to do something similar in his previous life, which is why he has confidence he can make such a fake or switch in this one. We shall see, it seems.