Ossuary

"A thick layer of ashes covered the walls of the place. Filth crept into every nook and cranny."

The Ossuary is a third level biome.

A red haze hangs over these chambers. Red sparks float up from below. Through holes in the walls, one can see enormous piles of bones outside, as far as the eye can see. More piles of bones litter the floor of the Ossuary itself, and deformed bodies and headless corpses hang from the walls and ceiling.

A Slasher elite can be found here who drops the Ram's rune. He disappears after being defeated once.

Access
A Spider Rune is required to access this biome through Promenade of the Condemned and reaching Prison Depths. Also, this last one can be skipped by using the passage with the Teleportation Rune from Promenade of the Condemned.