I have to say the underground city plot read much better after waiting for it's completion and rereading.
Granted it has tons of narratological and pacing issues but it helped establish the threat of the black calamity in a way that actually feels "threatening" (even if it was underperforming in explaining the actual origin of the crests). Now the logical following is to understand how humanity on the surface survived (maybe people outside Vella zone of influence, IIRC it was stated that Vella wasn't the only power of the time) and the calamity got sealed (old cave king after the meteor strike weakened it, another party, etc), something that probably link into Hiel's kingdom foundation myth and the consecration of the royal family.