I actually just re-read chapter 20 last night because I needed to clarify a timeline detail for a theory I'm working on. This is the chapter where the situation in the Undercroft spirals from bad to desperate. The main crew, after their narrow escape in the previous chapter, is hunkered down in that makeshift safehouse. Rynn's wound, which everyone thought was just a scratch, starts showing signs of being tainted by the miasma from the creatures, so you get this tense, claustrophobic sequence of Kael trying to treat it with limited supplies.
The big event is the arrival of the 'Whisperers' – those gaunt, humanoid things that communicate through overlapping voices. They don't attack directly; they just surround the building and start their chant, which slowly drains magical energy and, more chillingly, starts to influence Lyra. We get a flashback from her, a fragmented memory of her childhood in the Grey Spire that suggests she has a dormant connection to this exact kind of corruption. The chapter ends on a brutal cliffhanger: Kael, in a desperate move, decides to use the forbidden resonance crystal they've been carrying as a deterrent, but the moment he activates it, the entire chamber's runes light up red, and the final line is something like 'The door we sealed began to open from the other side.' So yeah, it's the point where their hiding place becomes a potential tomb.