Eberron

Eberron was a planet somewhere deep in the Ethereal plane with its own unique cosmology.

Cosmology
Eberron was a part of the Great Wheel cosmology and the overall multiverse, being situated in the Deep Ethereal. However, there it lay at the center of its own material plane and its very own cosmology of other planes that orbited it and had profound effects upon it. Furthermore, this cosmology and its Ring of Siberys sealed the world away from the regular Great Wheel planes, preventing outside deities and powers both celestial and fiendish from having an influence there.

Nevertheless, it was possible to travel to and from Eberron via the planes, at least in theory. One could go through the Ethereal plane or use one of the various spells intended for traversing the planes, but the cosmology was designed specifically to prevent this. Alternate material planes such as Eberron's might also be reached by going deep into the Plane of Shadow or through a shadow portal there, using a shared demiplane as a "corridor between worlds", or even as a freak result of cataclysmic planar event like breaking a staff of power, but again this was only theoretical. Only three very specific methods of getting there are known.

First, there was of course the World Serpent Inn. A variety of doors here opened to Eberron, with one of the few known relatively permanent ones leading to the Queen's Kiss flower shop in Fairhaven. Others opened to Sarlona, Sharn, and Stormreach, while one rotting door went to a ruined city known as Metrol.

Second, there was a way through the Demonweb Pits. Starting from Toril, a cave south of the town of Eveningstar, Cormyr, contained a portal to a part of the Demonweb Pits near the Rift Between Worlds, which linked to the Spinner's Prison in Khyber, Eberron's counterpart to the Underdark. From there, one could enter the Harbor of Stormreach, a city on Eberron. One could return the same way and afterward teleport to speed the journey. This was only possible after the Spinner's death in 1479 DR.

Third, there was a way through the Demiplane of Dread, otherwise known as Ravenloft. A misty portal between two trees in the House Jorasco Enclave in Stormreach took a traveler who heard the cries of anguish emanating from within into the mists and onto the Old Svalich Road in Barovia. A similar portal also stood, conveniently, near the same cave south of Eveningstar. Certain portals in Barovia took one back to the Eveningstar cave or the House Jorasco enclave.

It's unknown if Eberron even had a crystal sphere within the phlogiston, and hence if one could fly a spelljammer there or not.

History
Eberron and its unique cosmology were created by three great progenitor wyrms. They designed a new cosmic system, made a new planet, and populated it with recreations of the dragons, elves, orcs, and other races of the multiverse. However, they intended that they evolve outside the influence of their respective deities like Corellon, Lolth, Gruumsh, and others.

In spite this, Lolth would still inevitably somehow gain influence there. After an imprisoned demon Overlord of Eberron known as the Spinner of Shadows (who seemed a lot like Lolth) was slain by adventurers there, a passage called the Rift Between Worlds opened in the Spinner's Prison. This led into Lolth's Demonweb Pits. Elminster, the Sage of Shadowdale, and the Gatekeeper druid Tavnar Kronnak investigated this rift from their respective sides and curious adventurers went through. They found taunting messages from Lolth suggesting she'd allied with and tricked Eberron's Lords of Dust and had drow servants there, as part of a plot to acquire dragonshards, destroy Eberron, take over Toril's Weave, become its new goddess of magic, and so on and so forth. She even encouraged the adventurers to come through, like flies into a spider's web. A nearby portal took them to Toril, to a cave south of Eveningstar in the.

It seems the world was lucky enough to have had a Last War. During this Last War, House Cannith built an enormous prison complex to contain alien and extraplanar threats for all time. A warforged named Warden was built to be the immortal keeper of this prison. On the Day of Mourning, the prison fell, and Warden became corrupted by the very things they were been created to imprison. Becoming a servant of the Elder Evil Hadar, Warden somehow crossed to Toril to procure souls for the Dark Hunger.

House Cannith hired the goblin engineer Lazaapz to investigate an eldritch machine built by the Cul'sir Dominion for an unknown purpose. Lazaapz discovered it had markings associated with the local plane of Dal Quor. Shortly thereafter, both Lazaapz and the device disappeared from Eberron and transported to the world of Toril.

In the late 15 century DR, during a celebration of Dragondown, a group of dolgrims from Eberron somehow made an incursion into Toril, plaguing the village of Skeefan and the forest around it, the Tarlain family estate, part of the Trackless Sea coastline, and the lair of the bronze dragon Chotyn.

Geography
Eberron was the site of such cities as Fairhaven (capital of Aundair), Sharn, Stormreach, and Metrol, which lay in ruins. Its continents included Khorvaire, Sarlona, and Xen'drik.

Inhabitants
The world was home to dragons, rakshasas, goblins, warforged, elves, orcs, and other races of the multiverse.

Background
This world is of course the Eberron setting introduced with the Eberron Campaign Setting for 3.5 edition in 2004.

A copy of Player's Guide to Faerûn can be glimpsed in in the Eberron sourcebook Sharn: City of Towers, cheekily suggesting the Forgotten Realms is a fictional world in Eberron.