Its favourable location of equal "subjective" distance between the Abyss and Hell make it the most active battle-zone in the Blood War, where Demons and Devils clash over who is the most evil. Unfortunately this distracts from what Hades is all about.
It is the inverse of Elysium, being the place of ultimate evil in the existence. Worse perhaps than the Abyss or Baator if only because it is not subject to the capricious whimsy of Chaos or the regulations of Law.
It is a bleak place of suffering and misery with no ulterior motive; therefore there is cruelty without being sadistic and selfishness without ambition. Put simply, Hades represents evil for its own sake.
As the purest Neutral Evil plane of all in the Great Wheel, it is the center of the Lower Planes.
Like Elysium, Hades has an all pervading aura that causes visitors to be ensnared by its nature. In this case the removal of anything joyous or worthwhile.
By merely stepping onto the plane, non-evil individuals will be subjected to harsh penalties to their mental faculties, which makes colours appear duller and sounds seem more muted. Emotions seem more alien, and visitors find themselves less empathetic or caring than they used to be.
Not only that, but the plane is also home to a unique malady known as "The Grays", which affects all visitors, including evil ones and even if they are outsiders. This steadily causes those afflicted to lose their mental faculties for every day spent abroad in Hades as they become more detached from reality and hope. In rules terms this forces their wisdom score to drop for each failed attempt to resist the condition, bringing their ability to think ever downwards until they lose their sensibilities altogether and go mad. Of course this is more dangerous for non-evil people since their senses have already been assaulted by the bleak nature of the plane, meaning they are more likely to be overcome unless they have wills of steel. The only immunity is offered to those who have high enough levels of spell resistance. However the cure is simple: get them away from Hades and their madness clears itself naturally.
The last defence of the plane is its entrapping nature, with every week spent on the plane slowly bringing visitors down to its level. This is the plane exerting itself on the better natures of mortals (Outsiders such as Demons and Devils are immune). Making them forget happy memories, or distorting their feelings into something twisted and spiteful. This of course is coupled with the influence the plane has on the senses and the affliction of the Grays, meaning the minds of people who come here will eventually crack. Eventually they forget what it means to have hope and joy and they become trapped in the plane, unwilling to leave and are more likely to descend further into madness if the Grays has not gripped them already.
As for the plane itself; it fits its nature quite well. Despite the fact that sounds and colours are already drained... there simply is no colour, the sky is grey, the dirt is grey, and the inhabitants and structures tend to the gunmetal ranges.
There is no sun, moon or stars, just an impenetrable grey sky that goes on forever.
The first layer (also referred to as a "Gloom") is pretty much a wasteland. There is not much here other than craters, the occasional husk of a tree, the forgotten detritus of ancient wars and a whole bunch of rotting corpses. If you need more inspiration, the realm probably resembles no-mans-land in WWI with all the colour and depth of a black and white photograph.
Oinos is the stage of the Blood War and thus you don't want to roam here too often, otherwise you'll find yourself running into Demons or Devils or worse.
But not only that, you'll have to look out for the Yugoloths, the Neutral evil faction of outsiders much like Devils and Demons. The seat of the Yugoloths power in Hades rests in Oinos, where there is a tower, twenty miles high called the Khin-Oin. The tower is said to be the spinal column of a long dead god that came to rest on the plane. At the top of the tower is a throne, called the Siege Malicious where the ruler of the tower may sit. It should be noted that the temporal ruler of the Yugoloths is the General of Gehenna, though his power only extends as far as his ability to enforce it, and since he's on another plane the Yugoloths here pretty much serve themselves.
The throne of the Siege Malicious gives the user the ability to create or modify any disease with just a thought. It comes with its own price, as any ruler who sits on the throne is caused to have their skin slough off and disfiguring them. But this is nothing compared to usurpers who sit on the throne without the right to do so, who are inflicted with a particularly severe case of the Gray Wasting which is yet another disease inherent to the plane, but unrelated to The Grays or the entrapment effect.
It is often said that Oinos is the origin source for all disease in the multiverse. So you don't only have to watch out for the Grays and the spiritual weakening that comes with Hades, but you'll also have to prepare for the smorgasbord of pestilence and corruption that exists on this layer as well.
This gloom is all the worst environmental features of Hades made claustrophobic.
The grey aura actually closes in on you like a fog, meaning your visibility is reduced significantly, as well as muffling sounds to near in-audible levels and the level of moisture in the air creates a sense of uncomfortable dampness.
The topography is of a thick forest filled with carnivorous creatures and roving monsters. So while you are less likely to walk into a battalion of devils or demons, you are more likely to be stalked by a predator that you cannot see.
However, the layer is probably more inhabitable than Oinos since it is not a perpetual battleground, and it is much easier to hide here; thus there are settlements here and there. Most notable is Death of Innocence, which is a small town of mortals where the inhabitants at least attempt some semblance of trying to improve their lot. For some reason the Grays and the entrapping nature of the plane have no hold here, so the town could be likened to anywhere else in the prime material plane... if not for the fact that the wood that the houses are built from ooze blood instead of sap, and the fact that anyone who actually wants to eke out a living in the plane of ultimate evil probably don't make for fantastic neighbours.
Beyond the Death of Innocence, several Neutral Evil deities make their realms here, mostly because it is convenient to do so.
The final gloom of Hades would appear the most peaceful. It is a realm of withered forests and dusty olive groves. It's not quite terrible, but the aura of the place is one of impending doom and of being on the brink of apocalypse. There actually is not a lot of activity here and you could probably cross the layer relatively unmolested.
Pluton is really the dumping ground for its entrapped mortals and petitioners. It is that "classical" afterlife of despair and self-pity. Unlike the more imaginative punishments of Hell, those who end up here are generally left to wallow in their own suffering in the desolate landscape... At least until a third party comes along. Devils and Demons come here looking for souls to conscript for the Blood War, whilst Hags occasionally herd petitioners together for use as currency.
Pluton is also the bottom of Mount Olympus due to its connection with the Greek pantheon, therefore if you're clever, just find a hill and keep walking upwards. If you're fortunate you might eventually end up in Arborea, but you're more likely to go mad while trying, as this place for all of its relatively quiet and peaceful nature still remains part of Hades and will still suck you dry of emotion and goodness.
There are two types of petitioners in Hades: The ethereal kind, who are disembodied souls who don't even get sensory bodies with which to suffer their eternal torment, thereby adding to that sense of loneliness and despair. These spirits tend to flock towards mortal visitors like desperate wastrels, hoping to leech off some of warmth and positive emotion, but they are harmless.
Then you get the larval kind of petitioner, who are common throughout the lower planes as giant man-sized maggots with mortal faces. Larva get used as currency, demon-food, materials in spellcasting, or as raw recruits for promotion into the higher fiendish forms such as Demons and/or Devils.
Due to the differences of religion on different worlds in the cosmos, there is no hard guarantee of which form you take in the afterlife of Hades. Though as a rule of thumb, particularly wicked and evil people become Larvae, whilst those who "end up" in Hades through lack of faith or by committing sins are probably going to end up as spirits.
Hades is also the home realm of the Yugoloths which are the Neutral Evil fiends. They never really get as much screen time as the Devils and Demons, if only because the other two are more easily contrasted with each other. These guys are just evil for the sake of being evil. Also despite this being their home plane, they are more common in Gehenna where they migrated en-masse because, in case you haven't been paying attention, this place is a shit hole even by hell-plane standards.
As a result of the Yugoloth's disinterest, the primary planar power in Hades are the Night Hags, a fiendish branch of the hag family who focus on herding and trading the larva, both as lone "shepherds" and as mercantile consortiums.