It's tough to write about the Great Cataclysm.
For one thing, it's technically still happening. Adventures in Kos are the story of the ongoing defunction of a once-grand and noble people. The other reason is, the Cataclysm affected physical, universal laws (such as time, causation and correlation, chemistry, physics, genetics, etc). Therefore, it is difficult to describe.
For example: in some places, the Great Cataclysm is still happening. It happened hours ago. The dead rose from their graves. The oceans turned to shrieking squalls of red blood as the mountains shattered.
Poets, drug addicts and evangelists went insane by the thousands. Every apocalyptic prophecy turned out to be true simultaneously. The men and women of those places are still fighting the war.
In other places, the Cataclysm happened 40 or 50 years ago. Pre-Cataclysmic technology is considered magical, and effectively is. Man has descended to a primitive state, little better than apes.
In yet other places, the Cataclysm was generations ago. Civilizations are beginning to rise, resembling the old ones in some ways but substantially different in others.
In still other places, the ancient monuments have dwindled long to lonely marble mounds. The dust that once composed them is carried by wind and wave to place unrecognizable by inhabitants of the forgotten former world.
Pre-Cataclysmic knowledge is largely mythical. "Sorcery" is simply a method of manipulating the illogical physics which compose this brave new realm, like a programmer hacking a computer. Bizarre and competitive new species have arisen, seriously challenging the future existence of mankind.
Most of Kos, as written it so far, takes place in one or two of the latter categories. The town of Pilgrim's Passing occupies a relatively neutral area, as does the majority of the Kos River Basin and the surrounding Wildwoods, Lands of Woe, and Harshwater on the West Side of the River.
However, Thrundunmun exists in such a situation that the Cataclysm is still taking place, and will do so forever in this place.
"I have developed a habit where I simply lie in my bunk at night. I'm not the only one who does, but I go in early. I lie and I look at the ceiling and I picture it as the sky. And I look at the stars that you see when you close your eyes, and imagine they are the stars, and I am looking at the sky, which we can no longer see. When I do this, I wonder if, in all of the coincidences that happen in the world, maybe you will be closing your eyes and doing the same thing, and looking at the stars inside your eyelids too, and - I know it makes no sense - somehow it will be like you and I are looking at real stars together again. And so I go in early, to increase the odds it'll be at the same time. I do it alot, actually; we don't sleep much any more. There's not much time between attacks and other things that occupy the days which have no sunrise.
It's temporary, though, like everything. The pain and fatigue are just temporary sensations.
It'll end, and it will seem like it wasn't that long ago, when we're together again."
Letter carried by Red Courier. Delivered to a location which was found to have been abandoned. Letter remanded to Secretary General of the Red Knights, Thrundunmunn.
Nort B. Unthbe, 33rd Red Light Legion"
One area is in fact actually the rotting carcass of an incarnate being of physical decay, which occupies a space the size of a peninsula (such as Cape Cod or Cornwall).
Other areas are actually infinite. The Endless Mountains, as far as can be ascertained via both experiment and magic, have no physical endpoint. Neither does the 7th Desert of the 7 Deserts.
Nor the South-Land Sea, to the South. The Eastern Mountain Country is poisonous to body and mind and thus considered forever unknown and unknowable
In an area like Pilgrim's Passing, the land called "Kos" is actually the area once occupied by a nation called the Kingdom of Kos, now shattered and irrevocable. The few poor survivors of the disasters and the predations of savage new inhuman inhabitants must now contend with a vastly different world. They are a hard people. Items and knowledge from before the Cataclysm are hoarded by the very powerful. "Magic", (understood as the language and culture of the Pre-Cataclysmic People) is in the infancy of understanding, and largely misapplied (with disastrous consequences).
People receive rare reports from other communities but far-off lands are a mystery. Yet, new religions have arisen from foreign lands to confront the ancient Kossick deities. The barrens and even the roads have fallen into serious disrepair and are flocked with brigands, lunatics, witches, hungry beasts and monsters of all description. They are not untraveled, but Merchants and other adventurers are often considered brave to the point of dangerous lunacy.
And yet, even as the White Mounds of the First Age wither into dust, there is yet no end. The society of Kos will continue to slip away, forever, always leaving tantalizing pieces but growing farther every year. This is the true Curse of the Great Cataclysm: it made itself permanent, except in the ways that the players change it. Those are not only permanent, but often permanent to all other players of Kos, and all interpretations of Kos, as long as I am the Dungeon Master.
In Pilgrim's Passing, the standard starting town, most people don't know this. Most people don't figure stuff like this out until they've raided a few dungeons.
The Underworld is literally the place underneath the world. It is a Hell, populated by montrously otherworldly beings and the severely insane. The Underworld is easy to find - go to the deepest place you know, and keep going down. In the Deep Dark, everything drips downwards - all of the evil, all of the liquid, all of the corpses, all of the hunger, all of the treasure, all of the strongest and blackest. At the bottom is the Grey Sea. The Grey Sea is where the Unworthy go when they die. It is a sea
made of their souls. It can be navigated and traveled, by certain measures. The places it leads to are terrible, and sometimes the terrible things in these places are able to get the World In Which We Live. These are called "Bleed-Throughs", "Intrusions", "Demons", "Spirits", "Sandestins", etc. They have a connection to the world of Death (they are always its vessels, unwitting or otherwise), and spread It everywhere they go. Some take on human flesh as a vessel; others exist invisible to most eyes, and still others take on bizarre shapes of their own devising and understanding. Some used to be human. Some philosophers conjecture that it is these creatures who, in fact, rule the Underworld. This may be. The only way to escape the Grey Sea and the Underworld is to die in an interesting way - in which case, Adventurers (who are heroes by virtue of occupation - they always die in an interesting way) ascend to the Hall of Heroes*
*Which takes the form of a piece of notebook paper with the name of the dead character written large in crayon, with the method of their death and perhaps a few details or fond memories. Occasionally they are adorned with extra distinguishment or salient comments from former comrades. It is whispered that sometimes, those characters can even be contacted through certain complicated spells.
The piece of paper is kept tacked to the wall of the room where we play D&D