Pandaemonium is a hall of evil spirits located at the center of Hell in John Milton's Paradise Lost.
"Where Limbo is a rioting whirlwind of nonsense and pants, Pandemonium is ALSO a rioting whirlwind of nonsense and pants, but now the pants scream at you."
The plane is madness and insanity given form and will grind your mental state down into depression and mental illness, though not quite with the bizarre randomness of Limbo where anything is possible and you never know quite what to expect from moment to moment; nor is the plane like Hades either, which weighs heavily on the mind and forcing you to become more spiteful and miserable. Pandemonium represents the sheer futility of actually trying anything constructive, and assaults the senses with confusing and illogical terrain coupled with tumultuous noise and activity.
Despite never seeming to give travelers a moment's rest, there is very little to do here and the plane is mostly devoid of inhabitants other than the insane petitioners and whichever gods of chaos and madness that choose to make their domains here. You could travel the windswept tunnels for ages and never come across another living being, though of course this just adds to the depressing loneliness and futility of spending any stretch of time wandering the tunnels. Yes there are settlements and cities here, but they are sparse and hard to get to.
It's a plane for the insane, who are better just left there.
Pandemonium is most well known for its howling winds and endless caverns. Like the elemental plane of Earth, the entire landscape is underground, like an infinite Underdark. Although gravity is subjective based upon which side of the cavern you're walking on. So you and your adventuring party could march down a tunnel but be standing at odd angles from each other, making for a disorienting experience.
The winds are what makes life even more intolerable though. The tunnels are constantly filled with extremely loud and strong winds that frequently throw up debris that can be highly dangerous to the unwary. Even when the winds are at their most calm, the only conversation that can be had is performed by shouting and spells that rely on sonic effects have their range limited to 10 feet. The level of noise will cause you to go deaf within a day, unless you were clever and brought yourself some earplugs or wander around with your fingers in your ears. Either way, the presence of the wind knocking you off balance makes it very difficult to accomplish anything, especially since you're already bumbling around in the dark with your fingers in your ears like a berk, barely able to see anything.
At other times the wind whips up dangerously, managing to lift rocks and boulders or even people who haven't managed to hold on to anything, and throw them back down the tunnels, knocking them about and causing major damage.
Finally, the River Styx begins in Pandemonium, starting as a trickling stream that runs down the cavern walls, it will eventually become a raging river that passes through the lower planes until it ends up deposited in the bottom layer of Acheron.
The top layer contains the largest caverns and tunnels, many of which would be large enough to hold cities or countries, if they were ever insane enough to live here.
There is one respite though; the Madhouse which is the headquarters of the Bleak Cabal, whose existentialist philosophy helps them resist the effects of the plane. The Madhouse is actually a large city set up in one of the larger caverns of Pandesmos, and is remarkably hospitable considering the nature of the plane, possibly even more tolerant and easy-going than anywhere else in the multiverse too. Mostly because the attitude of the Bleak Cabal is quite selfless in spite of (or because of, depending on your point of view) its gloomy outlook and disbelief of inherent meaning in the multiverse. Like in Sigil, they spend much of their time administering comfort and aid to the insane petitioners and sick-minded people that somehow find their way to the city, so it is quite literally a "mad-house" akin to an insane asylum.
The second layer has the odd distinction of appearing hand-made. Where the the first layer is an endless labyrinth of natural caverns, Cocytus looks like the rock had been mined or carved in some distant age long past.
The tunnels and caverns are smaller in the second layer, which causes the wind to concentrate into harsher speeds and sounds, making the place even less tolerable for habitation.
There are a few points of interest though, one cavern known as Howlers Crag has a pile of rocks in the center of it as if a fortress had collapse on itself, rumor has it that if you stand on top of the pile, any message you shout will be carried off on the winds to its intended recipient, no matter where in the multiverse they are.
There is also an odd magical location called the Unseen Path which is a section of tunnels laid with cobblestones, but is entirely shrouded by the effects of permanent deeper darkness magic. If you can walk the entire path you get improved hearing and darkvision for a year, but the difficulty is that unlike the rest of the plane, the pathway has fixed gravity and twists and turns in the darkness, meaning a missed step could cause you to "fall" into the ceiling or a wall, spoiling your progress and forcing you to start again.
Another notable landmark is the "Harmonica", a giant spherical cavern filled with spires of black stone. Nobody knows who made it or what its purpose is.
Hruggek, a bugbear god, has his realm of Hruggekolohk here.
Unlike the other layers, Phlegethon has fixed "downward" gravity, meaning you can't go running down walls or ceilings, but it also means that water flows differently, and while the winds are slightly more muted in this layer, they are accompanied by an annoying dripping that is filled with foreboding.
Not only that but the layer absorbs light and heat, meaning magical and natural effects to do with light and vision have a reduced effect. It also makes the place uncomfortably cold.
The final layer doesn't consist of tunnels, but rather pockets of open space that lie unconnected with each other. Arguably it's not a layer as such, but the bubbles in the solid space of Pandemonium itself.
That means if you find your way into one of these gaps and don't have access to plane hopping magical powers, you are FUCKED, unless you can find the portal out, but then of course you'll need the right portal key. Good luck with that!
For that reason, Agathion is used as a hiding spot for various artifacts or creatures that the multiverse would rather not get into anyone's hands. If you put an object in a cavern pocket and then conceal the portal to that cavern, it's likely never going to be found. Sure a Dwarf might say they could dig their way to buried treasure, but we're talking about infinite distances of solid rock where direction holds no meaning and howling winds battering your every action and draining your willpower. Nope, if something is stuck here it's never getting out unless whoever put it here wants it out.
Dead Gods stuck the Wand of Orcus here, and involved the PCs in getting it #canceled for good. But, well... Dead Gods.
While all outer planes have interesting native inhabitants, (pandemonium for example has white four-legged things called howlers and beetle-things called Murska that shapeshift into the last thing they ate), the only notable intelligent inhabitants of Pandemonium are the petitioners, those unfortunate sods who likely went mad in life and so spend eternity walking the tunnels of the plane in their madness. A few gods make their homes here, so they will likely have the largest concentrations of petitioners living with them, so you're still unlikely to encounter a random petitioner while travelling between realms. Being infused with the nature of the plane the petitioners aren't really affected by the wind either, so they're not always deaf, though it's unlikely they're going to be listening to you if you try to engage one in conversation.
Mage: The Awakening
In White Wolf's Mage: The Awakening, Pandemonium is one of the five Supernal Realms a Mage can project their souls to upon their Awakening. Pandemonium is the Realm of the Arcana of Space and Mind, creating a confusing, twisting dimension in which will determines motion and all your flaws, vices and nightmares come to life as demonic entities. A Mage who signed their soul on the Watchtower of the Iron Gauntlet becomes a Mastigos, a Warlock on the Path of Scourging.