In the dwarven holds, acoustic engineers known as song-smiths map unmined rock by singing specific frequencies and reading the echoes that return. Kaelia Iron-Chord is the best in Oakhaven Hold — precise, analytical, more comfortable with the perfect mathematics of acoustics than the messy politics of dwarven society.

While mapping the lowest stratum for a new expansion, she sings into a darkness no song-smith has reached in centuries. The echo that returns is wrong. It changes pitch. It responds.

Vane Stone-Echo is a disgraced tunnel-warden carrying the trauma of a squad lost to what everyone called a cave-in — but he knows it was an attack. When he hears what Kaelia encountered, he's the only one who believes her.

As the creatures from the Deep Roads breach the lower levels, Kaelia and Vane must use every acoustic trick they know to corral horrors that communicate and hunt through seismic vibration. But the creatures aren't just attacking. They're adapting.