The Elemental Chaos was an Inner Plane in the Great Wheel cosmology, located at the far reaches of the elemental planes, where the elements dissolved into a single mix of colliding energies. In the World Axis cosmology, it was the fundamental plane within which were contained all of the former elemental planes and energy planes, as well as the planes of Limbo and the Abyss.
A tempestuous sea of ever-changing terrain and clashing elements, with stability only enforced in isolated locations, the Elemental Chaos was ruled over, at least in name if not fact, by the primordials and was the equal and opposite of its divine counterpart, the Astral Sea, as the primordials were the opposites of the gods as demons were to devils.
In spite of its chaotic and dangerous landscape, most parts of the Chaos were relatively easy to travel through, or at least as much as most other planes.
Elemental Realms
When the elemental and energy planes collided to form, or possibly reform, the Elemental Chaos most of them were lost, along with the other planes that fell into the mix, such as Limbo or Jotunheim. However, within the Chaos those beings strong enough and powerful enough to shape the materials around them were able to create new planes or preserve parts of old ones, either of which became known as elemental realms. Here numerous elemental creatures resided, alongside the mighty primordials who first created the planes alongside the gods.
The known elemental realms are listed below:
The Abyss
The Abyss was the home of demons and was ruled over by none, though several of the powerful demon lords each claimed control. A multi-layered pit, the Abyss laid at the very "bottom" of the Elemental Chaos, waiting to draw in and ensnare any who came too close.
The City of Brass
The City of Brass was the home of the efreeti, fire genies who long ago forged this city, which was believed the oldest in all the planes. Though efreeti were known for their wars against other elementals, the City of Brass was best known as the center of interplanar trade.
Cresting Spires
Dominated by the primordial Istishia, the Cresting Spires were a plane dominated by water.
Fimbulwinter
Forged by the titan Thrym, this plane was covered in ice.
Hidden Realm
The most powerful of the titans, Annam, ruled this mysterious realm.
Muspelheim
Ruled over by the titan Surtr, Muspelheim was dominated by a volcanic landscape.
Root Hold
Dominated by Grumbar, the primordial lord of the earth element, Root Hold was a series of vast subterranean halls with strong connections to the Dismal Caverns and Dwarfhome.
Sky Home
A realm ruled over by the primordial Akadi and dominated by rolling clouds and dark storms. Sky Home had links to the plane of Arvandor.
Steading
This realm resembled a landscape of rolling hills and was dominated by the titan Grolantor.
Thraotor
A plane of storms this plane was ruled over by the titan Memnor.
The Undying Pyre
The Undying Pyre was a fiery plane of thick smoke and mighty flames, ruled over by the strongest of the primordial lords, Kossuth.
Zerthadlun
Zerthadlun was a realm shaped by the minds and will of the githzerai, mortals who had taken residence in the Elemental Chaos in order to study the entropy that filled it. Through constant diligence and monastic ritual the githzerai maintained their realm, in spite of the chaos that threatened to engulf it.