Hades

Hades is an outer plane that is pure evil with a perfect balance of law and chaos.
Hades has two layers to it which are referred to as grays or glooms. These glooms are completely devoid of color, emotion, and hope. If one were to enter the world with something with a vibrant color, be it organic or material, within a week it will lose all color and turn to a gray even if it was brought back to another plane afterwards. If a mortal being from the material plane were to enter this plane with anything but a powerful divine protection, they too would succumb to the gloom. First, all emotion would drain from them within days leaving nothing but sadness. Already the being would have no desire to leave as they would be unable to muster the effort to do so, within the span of two weeks, depending on the beings' protections and strength of will, they would turn into husks or hollows. This is referring to a being is not dead but devoid of any soul as they wander aimlessly or collapse onto the floor waiting for death. Eventually, their bodies and soul would wither away into ash that would blow into the wind, forgotton and unclaimable by any being.
Oinos
Oinos is similar to Avernous in the first layer of the nine hells. On this layer of the Gloom, constant battle could be heard from anywhere and nowhere was safe as demons and devils were constantly threatening one another. The Blood War that still lasts and will forever go on, has created mountains of corpses of all manner on this layer creating a putrid stench that was inescapable and a disease commonly known as wither rot. Wither rot is what is believed to be the main cause of the Husks/Hollows of mortal beings that for one reason or another, lost their way. This layer is believed to be the source and end of the river Styx as currents congruently flow out of and into a country sized ocean at its center. The rivers here flow much slower which is why there is more traffic and ferry's traveling through it mostly by deamons and devils attempting to cross planes to get to each other's land. Around the entirety of the massive expanse of the Styx are stone fortresses occupied by many daemons and other powerful creatures that guard the body of water fiercely and without mercy or question. The reason as to why they guard it so is completely unknown to all but those that do it.
Niflheim
Niflheim was the second layer of the Gloom and had very keen and drastic differences with the layer above it. Firstly, and most importantly, while still gray in color and a desolate aura that would sap all emotion but sadness from a visitor, there was no Wither rot here. Another distinct difference here was the wide spread of vegetation and forest like terrain. Finally, there is a thick layer of fog that permeates through the layer that even those with enhanced sight would not be able to see farther than about 100 feet. This fog also has certain qualities that dampens sounds making anyone but the attuned easy pickings for entities looking for a kill. Additionally, this fog also creates allusions and sounds of its own that can resemble a memory, many have died simply from exhaustion attempting to chase a person that wasn't there.