I mean the flashback you get after the ritual, Arvo telling his friends to not talk with Mikko without explaining why. You can learn about it from Mikko's route too, but in Klaus's route you can actually re-live that exact moment.
It's sad how little it took for them to abandon Mikko - just "believe me, okay?" was enough. No shit Mikko hightailed out of there and applied to a boarding school.
I wonder what this ritual moment even is - is it metaphorical? Is magic canon? Did Klaus put something into the dish he helped Dan cook? He said with a grin that he cut the tomatoes for it and I've seen people refer to home-made drugs as tomatoes(since tomatoes are the most popular thing to home-grow). No one else was affected, but it could be the bean lottery - Klaus could very well be aware of that tradition, and with his fascination with fate... Yeah, he probably drugged Arvo.
p.s. I too am not amused with some of the special artworks for this VN. I'd rather prefer if all artwork was done by HQ - their works really look like windows into the world of Dawn Chorus and not just fancy paintings. The simple picture of Arvo looking at himself in the mirror has more weight than any other art in a different visual style, IMO.