The Forest of Dusk, also known as The Waiting Place, is a wooded area in the Martial Empire. It is where lost souls go before moving on to the afterlife. It is the home of the Soul Catcher.
The Forest is full of large trees, some as wide as houses. The air is heavy with the smells of pine needles and moss-softened bark. It is dark below the canopy of trees. There is a constant murmur of spirits and ghosts.
The Forest is a sacred world, a place the dead come to find peace. It is sentient and the oldest magic there is. There is an invisible, glimmering wall bordering the Forest. It keeps the spirits in and the living out.
Inside the Forest is a grove of trees. These tress are where the jinn have been imprisoned. It glows red and is darker than the rest of the forest. "The trees are enormous, interconnected, and wrong. Their gnarled trunks glow as if consumed from within by the flames oof the hells." While they are locked in the trees, the jinn still have power within their small space. They use it to get into Elias' mind and mess with his thoughts. They also use their power to torment and mock Shaeva. They use their power to torture Laia by showing her the truth about her mother. The jinn groove is a vast plain on a bluff high above the Sher Jinnaat. Beyond the dark sprawl of the eerily silent city is the River Dusk. After the jinn were freed, the few remaining trees are blackened husks.
When Elias becomes the next Soul Catcher, Shaeva tells him: "To be Soul Catcher, you must know death intimately, but you cannot worship it. You must have lived a life in which you wished to protect other but found that all you could do was destroy. Such a life instills remorse. That remorse is a doorway through which the power of the Waiting Place can enter you.” 
The trees of the Forest change to reflect Mauth's moods. It is a physical manifestation of Mauth. To an outsider it appears as any other forest would, but Mauth alters it to fit Elias' needs.