Yooo, there's A LOT to unpack here, methinks:
1. Vandals became more dangerous (organized?) a few years back and started engaging in some experiments. Somebody united them maybe? Or at least united a few teams under one goal and banner? That way, we can expect a rising force, an army of sorts, slowly preparing to strike us for example.
2. Alright, so... they need and want Skypeople. "To talk to them", they claim. Do they? If they do, why? What do they want to know? Do they want to usurp heavens? Bring it down, invade it? Are they also looking for the way up, like Rudo does? Or MAYBE they're experimenting on Skypeople instead? Maybe they have something different in their genes? Are they resistant to some kind of disease of the Abyss?
3. Soooo, we knew that Rudo is rare, but Vandals seem to make him even rarer. I personally thought Rudo woke up alive due to those lines holding him when he was falling, but maybe that's not the case? What's the border? Is THAT why they rarely see Skypeople in the Abyss? Not because of aberrant beasts and not because of human trafficking, but due to this border, possibly hard to cross?
4. Is Rudo special then? After all? I don't really like "special", be it bloodlines, prophecies and other disgusting BS, but gotta let Urana-sensei cook.
5. Nijik, nooooo. I loved that about his stick and his familiarity with it, but it really seems like that boring trope of "nice senpai wrecking some randoms and then immediately getting wrecked by protagonist's next opponent". Please, no, at least let's write it a bit better ^^