Spelljammer 4th Edition
The Elemental Chaos
They say the elemental chaos is “beneath” creation. That is more of a figurative term, but some say that there are various yawning portals in the deep places of the world that are in fact chasms into the infinite sprawl of the Elemental Chaos. The elements float and churn in a sort of cosmic stew on this plane. Gravity functions as normal, however for animate creatures and elemental masses not native to the plane. One can pick up a rock in the Elemental Chaos and lift it in the air, and the rock will float.
The realm is also very unstable. One can exert changes or invite stability on the raw stuff of creation with willpower alone. Particularly strong willed individuals have carved out entire domains within the Elemental Chaos, like the City of Brass or the lost cloud city of the Djinn. (if it truly exists.) There is entire cities of Genesai, and ancient strongholds used in the Blood War. Gyth Monastaries and Efreet outposts dot the realm.
Among all the enemies one is likely to encounter in the Elemental Chaos (There is indeed a great many) the unpredictability of the plane itself is a reliable and relentless foe. That said, any journey within the Elemental Chaos is better off done via Teleportation or Portals when available.
While spelljammers have relatively easy sailing in the chaos, avoiding the storms and colliding motes that erupt in superheated rock is a bit harder to do. Colossal flying Chaos Ships are the true gems of the Planes below, with their Anarch Spheres taking advantage of the planes elemental natural elemental buoyancy for propulsion. Entire fortresses are erected on the decks.
The plane is also the final resting place for a great many gods and particularly primordials. Some are buried. Some remain untouched. Some are still alive, frozen, subdued, or otherwise incapacitated, waiting for the day to be released.
The Pandemonium Stone
One particular oddity of the Planes Below, and some say, a dark omen, is the Pandemonium Stone. This great hunk of ….something appears randomly from time to time in the Chaos. Slaad are drawn to it, gathering in great swarms and “singing” to the churning mass in eerie cacophonous croaking cadences. There are two outposts on the stone itself with entities dwelling within that appear and disappear with the stone.
It is said that the mad god Tharizdun planted the Seed of Evil at the bottom of the Elemental Chaos, and it sank into the realm, spreading out like a drain into a chasm of horrors and torments and eventual annihilation. Demons don’t just embrace chaos for chaos’ sake. They are the evil at the heart of chaos. They want nothing more or less than to destroy all of existence.