Just when I thought "oh, a nice little exposition chapter to chill out", here comes the boss, in some kind of trance, mumbing plot points I didn't even know of xD Let's look into this:
1. Since we've only heard of "series" in conjunction with Rudo's gloves, we can assume that it's the Watchman series. Now, either there are more gloves or the whole set, like clothing, a weapon maybe etc. Why would it be important? Either due to Vandal's obvious interest in the Heavens or (and?) it's connected to the other 2 places.
2. City of Arts tells me nothing, but I'm pretty much making a mental note. We never got a proper geographical worldbuilding before, so I'm just happy we might get some. For all that we know, their city could somehow be this City of Arts, but doubt it. I'd assume that a "City of Arts" in a shonen manga has to be SCREAMING art on ever corner.
3. Desert altar... some kind of ritualistic shit? Religious maybe? Does the altar require a gift or sacrifice? Is Watchman series needed to be offered on the altar? What does the altar do? Open something? Some closed doors, a way forward, this enormous structure reaching the sky? Or maybe that's where the Vandals' found some traces and informations about Skypeople and Heavens? Not the "next step", but maybe the "first step"?
4. I got curious about the symbol we were shown. First I though - HEY, were they a Janitors too in the past, just another organisation, fallen since? Then I was like "Will this be important in the future and that's why it has half a page of space here?". Then I went back. Symbols are important after all... Remember when Rudo bumped into Legto's killer and that suspicious book fell out of their hands? It had THE SAME SYMBOL!
5. Now I'm even more sure, that they book belonged to Legto before and it was stolen, maybe the whole murded was due to the book. Was Legto somehow connected to those Vandals? We never knew how he got his money. Obviously he lived in Heavens, but maybe was kicking it with the Leader in his youth? Or maybe it's a bit more complicated and those Vandals' are kinda inspired or moved forward by whatever this book was about? Like, you know, indirect clue.
6. That's another connection between at least the Leader, but probably all the Vandals' and Heavens. It makes even more sense now for them to reach for Rudo. My mind is blown a bit - I love how packed those sneaky little chill chapters are <3