An Ergastulum is a building used to hold in chains dangerous slaves, or to punish other slaves. The Ergastulum is subsurface and large enough to allow the slaves to work within it and containing narrow spaces in which they sleep.
A test of transitioning to the "Mirror" dimension
The sound is really excellent, I love it. It is so quite and desolate, very uncomfortable, makes the sound you doo hear all the more disturbing.