Silent Construct

Silent Construct is a region exclusive to the Downpour DLC, with being the only Slugcat that can access it.

Overview
The Silent Construct is the decayed remains of Five Pebbles, long since fallen after having been completely consumed by the Rot, with most of his machinery silent and dead. After his collapse, his superstructure crushed the Shaded Citadel, destroying most of it.

Despite this decay and devastation, Five Pebbles himself is still barely alive.

Frosted Cathedral
These cold ruins are all that remains of the Shaded Citadel. There's light in the once-dark halls. Coalescipedes are no longer found in it, but s and s are still around.

The Husk
The Husk is what remains of Five Pebbles' superstructure, which broke into pieces when it hit the surface. Even the Rot is dying, to the point only a few active cysts and remain. Its once lifeless and tumorous halls are now teeming with life, both with the lizards that lived in the Exterior and the mice, spiders and centipedes that survived the Shaded Citadel's destruction.

Five Pebbles
Five Pebbles' chamber still stands despite the ravages of time. His mind barely active and his cloak ragged, the only thing he has left by this point is his music pearl, its contents distorted by entropy.

Trivia

 * Silent Construct has the acronym CL in game files, which stands for Collapse.
 * Given the deplorable state of the Rot by the Saint's time, it seems like it cannot survive on its own if the correct conditions of an Iterator's superstructure are unavailable, degrading and ultimately becoming inactive.