Chasm is a demiplane of an infinite number of deep canyons. It was defined in the Mentzer-D&D module, M5 - Talons of Night, pp. 38-9, by the erstwhile illustrator Paul Jaquays. Chasm hosted a segment of the path from Mystara to the Isle of Night, the latter a demiplane associated with the Sphere of Death. Chasm itself was somewhat necrotic and not meant for mortals.
This demiplane has always intrigued me, but it was difficult to see how one would actually base a campaign in such a hostile place. Recently, though, my brain got back in gear and I had a few ideas for porting this wonderful place into mainline D&D.
There are spoilers. Do not read further if you are playing in M5 as a PC. I'll also add a commentary, but I'll spare you that boredom at least until the end of this page.
In the elder days of the lower Outlands, child Chira Lan played melodies for the gods in the gilded bamboo forest of Teranith. While Chira Lan played, her music inspired the gods to create green worlds around golden suns, paradises for their peoples to dwell in. While Chira Lan played, the mortal peoples erected temples of jade and obsidian on their new worlds, rejoicing in the verdant beauty of Creation. And while Chira Lan played, even the demons and the gods of evil would pause in their shadowy tasks, if only for an hour.
One day Klalo Kee the Trickster went walking the woods of Teranith. And as he walked the dulcet strands of Chira Lan's pipes came drifting through the bamboo. And so Klalo Kee came upon Chira Lan in her circle of moss and listened, enraptured. When child Chira Lan finished her melody, Klalo Kee wept and spoke to Chira Lan, saying: "o child Chira Lan, your music fills the planes with love and beauty! o child Chira Lan, your music drives all evil to the far corners of Creation! o child Chira Lan, why ever cease playing?"
And child Chira Lan looked on Klalo Lee the Trickster, and a tear formed in each sapphire eye. "o clever Klalo Lee, I am but a child and not a goddess; I have breath but not of infinite store. Even the crickets and nightingales must cease their song when they tire."
And Klalo Lee the Trickster said to child Chira Lan, "Such is the doom of all mortals, even the deathless ones who live in the Outlands, this I know. But you have the art of fashioning as well as that of playing. I have found a thing that provides wind without resting. And if you could make it sing as you do your pipes, you would drive away the taint of darkness forever; and of those entities that are now the demons and the gods of evil, even they might create green worlds and golden suns, and cease anymore to be demons and gods of evil."
And child Chira Lan jumped with joy and clapped two hands like hummingbird wings. She exclaimed, "o clever, wise Klalo Lee! For such a gift I would do anything, even lay down my own life! Tell me where is this thing that provides wind without resting?"
And sly Klalo Lee told child Chira Lan, "I speak of the Tunnel of Feywind, deep in the dark cracked rock of the Walls of the Abyss, beyond the Howling Cave." And Chira Lan blanched, for she had heard of it; a round and infinitely long cavern, like the worm-holes that sometimes twisted the wood of her pipes, with an eternal gale that screamed like the voice of a storm through the stalks of long-dead bamboo trees.
Child Chira Lan pleaded with Klalo Lee the Trickster to find another such place, which had wind of that scale, but yet did not cease. But Klalo Lee told of no other. And so child Chira Lan entered into the Howling Cave even to the Tunnel of Feywind, and carved notches into the walls, so that they blew eldritch notes atop the howling of the winds before. But for every mile of tunnel she carved, another mile lay beyond. And Klalo Lee the Trickster turned back to the Outlands and plotted new tricks to play upon the naive.
For one year, the gods and mortals no longer heard the music of Chira Lan, and their joined works of creation and worship did pause. In the year following, even the souls of the demons and the gods of evil felt empty, with that last spark of purity lost, and fell to their wicked work with the added viciousness of despair. And in the year after that, the gods met on the slope of the Nexus Mount, which no god may climb, and spoke openly of their loss.
And one small demon, QuaTchak the quasit, did speak out at that conclave, saying he had seen Klalo Lee the Trickster and child Chira Lan together at the entrance of the Howling Cave. And the gods hastened to the Howling Cave, and heard it whistle as it howled, and some noted the strands of Chira Lan's melody in the whistle of the cave. And Raka Yaa the god of eagles saw the notches on the wall, and Siri Fo the goddess of music picked up the discarded pipe that lay to the outside of the cave. And the gods knew.
Of the Hunt for Chira Lan, is it not written in the ...? Many gods and mortals searched the carved tunnels, and those tunnels which still went uncarved; and many perished, or went mad, or disappeared into the Tunnel of Feywind. And ever their call for child Chira Lan flew apart in the whistling winds, with not even an echo in response.
At last they ceased their endeavours, those that could; and they seized Klalo Lee the Trickster; and they cast him into the Cracks of Sanity of the Walls of the Abyss, not far from the Howling Cave. A spell of coldest ice they cast into the crevice after him, and then that crevice they sealed. But none know if the spell sufficed to imprison Klalo Lee; and to this day men of arctic climes claim the Trickster comes with the blizzard and the howling storm, haunting the edges of mountains and seas. For those are ever the months of treachery.
But of child Chira Lan, none claim to know her fate, and none have found her in the twin Tunnels of Feywind: carved, uncarved - much less carving. Some believe that she will finish her task, and when it is done, the Howling Plane will become the Plane of Song. The mad will be sane, the deaf will hear, and thoughts of evil will be forever purged from even the lowest of the Lower Planes. In those days perhaps even the Trickster will find less destructive amusements.
-- The Child and the Trickster
The Child and the Trickster is mostly interesting for its claim as to the subplane Cocytus, namely, that it is not a "true" subplane but merely a finite, if not yet bounded, portion of Pandesmos. (Of course, by that logic, the four mounts of Gehenna are not true subplanes either, for they drift in the same space. But that's more interesting to Guvners and other such pedants than to me.) In fact I doubt even the Powers know what carved the whistle into Cocytus.
This tale also claims that the "Walls of the Abyss" - what we inaccurately call Pandemonium [lit. "Land of All Demons"] - contain cracks, when in fact the four known levels have only tunnels and bubbles. The Trickster is believed to be in Pandesmos in any case; why another, unprecedented level is needed for him is beyond me.
I include this story here because some manuscripts [of the Book of Inverted Darkness] contain it. NB: That is true for many such stories, and many of those are forgeries from any manner of fools with a pet theory about the multiverse, most of whom would not be seen dead in the planes concerning which they so wildly speculate. (The remainder ARE found dead in those planes.) Even if the tale were authentic, it is no proof of accuracy. But I am here to judge the former alone.
-- excerpted commentary from Pihnmid's Translation,
with square-bracketed editorial notes
Like most of the rest of Pandemonium, Chasm is dark and windy, and "down" is oriented relative to one's specific location. Unfortunately here that direction is always at a ninety degree angle to solid ground. One is always either clinging to a two-foot-wide ledge, or giving oneself up to the mercy of the wind. The top and bottom of any given canyon breaks (after a wretching perspective and gravitational shift) into the side of another canyon - no canyon has a ceiling. Therefore the walls of any given canyon can also break as if wounded by the irruption of another canyon. The walls of a canyon can be any distance apart from each other, and need not be parallel, although eventually they will end, either in yet another canyon or in a blank wall.
Consider a block of glass. Hit that block such that the insides shatter but the glass remains in place. Now imagine the glass is infinitely large, that the cracks are subject to change without warning, and that you are a fly trapped inside it; and that will give you an idea of how this spectacularly inhospitable layer is organised.
The wall is not entirely blank from top to bottom. There are the openings to impossible new canyons as mentioned above. There are ledges that lead from one point of the wall to another, even to those other canyons. If one falls off a ledge but is tethered, Jaquays ruled 2d6 damage for smashing back into the chasm wall. There are also occasional waterfalls, leading up to gates to other layers and planes. 'Course, those other places are typically unpleasant, so one must beware the water's effects.
The weather here is chaotic and violent. The wind follows the rules of the 3e Manual of the Planes table 7-3; note the Stygian reference may be optional (see below). If one is on a ledge, "dashes against rock wall" means "is thrown off the ledge". If one is flying untethered, ignore Jaquays's rule that the PC is permanently lost. Instead roll up side effects on table 7-3, and if it is a physical result, the PC is blown off course by that die roll times 10 feet.
On top of this, the winds can bring a storm over from a neighbouring chasm, smiting fliers with lightning and sweeping climbers away in sheets of water.
Chasm is also subject to the groans and cries of Enesidaones the Shaker. When an earthquake occurs, roll d100. That number, divided by 10, is the quake's strength. If you have a laptop, you can roll d10 ten times, to get a number from 1 to 10,000,000,000; and then calculate that number's logarithm base 10 and add 2. The Richter is a logarithmic scale. The ledge is subject to fall at 8.0, surrounding rock at 9.0, and at 10.0 the chasm snaps shut. Goodnight! Smaller quakes are more likely, of course, and they can do anything from kick you off the ledge to raining rocks on you. Earthquake effects can be found in the 1e Wilderness Survival Guide.
Things will grow on the walls, and storms do bring water, but it's all very unpredictable. Even the water can be tainted (see below). Also unpredictable is whatever may fly by outside - typically valuables (and occasionally corpses) of long-dead berks who fell off the edge decades ago.
This place is such a backwater that it has no name. "Chasm" is only used by natives of Thorne, itself only known to a couple of primitive nonhuman tribes in Mystara.
The following apply not only to Chasm, but to the entire plane.
I don't know if the Styx flows here or not, but that does not mean the Styx's influence is absent. The 3e Manual of the Planes p. 97 has it run through Pandesmos but doesn't clearly state anywhere else. However, the 2e Planes of Conflict make it clear that the Styx flows to if not through every later. This means that every body of water has the potential to be tainted with Stygian poison, of which amnesia is one of the more hoped-for effects (c.f. the Clearest Water in Thralhavoc's former domain).
If the Styx or related waters aren't on a given level, being "blown into the Styx" could just mean "blown into another sector", which in Chasm means another canyon.
Concerning the windy parts of Pandemonium, including
Pandesmos and Cocytus, and probably Phlegethon too, Talons of Night makes the excellent point
that in this sort of wind,
when the party arrives "each character must make an average Strength/Dexterity check to hold onto any
shields, capes, ornate helmets, or similar items with high wind resistance and minimal restraint.
Only one item per person can be saved unless the players outline reasonable procedures to prevent fly-aways
".