A silica planet at the galaxy's edge — a molten-cored sphere of glass whose crystalline natives "rise up from a crystalline ocean and communicate in song." Named in Right Hand Man; first visited in Overtones.
Vesper pitches it as "an amazing one made entirely of silica on the edge of the galaxy," where the people "are literally one with the planet"; Hugo's private notes wave it off as a "nonsense planet — highly superstitious and delusional." When the group actually arrives in Overtones, Hugo's account is less kind: a "living ball of glass," a perfect sphere "molten in the middle and hard on the edge," so walking it feels "like walking on a white-hot sheet of ice." There's no atmosphere — Eira seals her helmet — and never any quiet, just the constant windchime "din" the natives call song.
Those natives are the Threnic: crystalline shards that swim below the surface and rise to mimic a visitor's shape and sing. One greets the group, and Eira nicknames it "Melody." They ferry visitors on gliding glass seats and hold the Sonorium, a days-long funeral rite the group attends — the real business of the chapter. Vesper's friend Clara, revealed here to be one of the Array herself, lives on Threnos; the promised visit to her is deferred.