The Twin Dimensions

The multiverse is a vast, endless place. There are dimensions and worlds that exist that, by all rights, should not. Seemingly, from Sigil, you can enter a version of Eberron that operates on one set of laws, and a version that operates on a different set of laws. How did this come to be? Why does the world operate on such fundamentally different principles? The truth of the world is far more complicated than anyone in Sigil realizes - and there are worlds that are near mirrors of our own that exist just beyond the Azure Veil.

The World of Toril - An Alternate History
Approximately 13,750 years ago, the god of the elves, Corellon, launched a fierce and devastating attack on the other pantheons of the world - seeking, in fact, to acquire dominance over the entirety of the world and acquire dominion over evil itself. In his attempt to gain dominion over the world, he stopped at nothing - no god was spared his wrath as he formed his alliances and drew his armies upon the world. Rumors persisted that Corellon's sudden attack and change in demeanor was caused by a powerful mage cabal formed of star elves who sought to prevent a greater calamity, uniting the world beneath a single banner - but ultimately, their folly was soon proven.

Eventually, the world itself was drawn into conflict - each of the gods found themselves taking sides in a war without end, as lines were drawn, boundaries formed, and mortals drawn into a conflict that seemed to have no cease. One by one, the conflict claimed the lives of mortals and lesser gods and goddesses alike until only those interested in fighting remained, save for the few who managed to flee to the isolated places of the world, such as Gruumsh and his people who fled to the isolated land of Laerakond. Eventually, a cabal of powerful mages and artificers drew together to put a stop to the madness - working together with a betrayer amongst the ranks of the gods (later revealed to be the rebel Lolth), they created weapons born from the souls of dead gods - weapons referred to as Godslayers. These machines were capable of massive destruction, and with their help, mortals were able to push back against the Gods who yet remained - reclaiming their lands for themselves, and finally bringing an end to the conflict, and to the gods themselves with the exception of the two 'good aligned' gods who yet remained, and their few remaining vassals who served beneath them.

For their role in the conflict, the star elves as a race were sentenced to exile by the Celestials that their treachery might never be repeated and further were severed entirely from the weave. Unfortunately, without gods to serve, the Celestials' own ambitions were soon left unchecked. Seeking to end 'evil in the multiverse', they invaded the hells, and the entire host of Solars (save for the high solar Zariel, the only to object to the invasion), found themselves slowly but surely changed by the nature of the hells into lawful evil nightmares bent on enforcing law and order in a godless world at any cost. Likewise, their worlds now invaded and their stations now deposed, the fiends fled to the now-empty Celestia, seeking to make it their own and driving out the peaceful guardinials that made it their home - and over time, the very nature of that place changed them - they now taking the place of the good-aligned defenders of the heavens. Such was the universe's way of ensuring that balance was, indeed, maintained. Over the last fourteen millennia, the world of Toril has slowly fallen apart, particularly its weave and magic, leading many scholars to believe that the disappearance of the gods has led to the ultimate end of the world. Mages, seeking to create a 'new divinity', succeeded only in creating a number of items called 'wish anchors', which ultimately failed to achieve their task.

Star Elves and Sildëyuir
Star Elves live in a dimension-between-dimensions known as Sildëyuir, capable of seeing the entirety of the material planes of the worlds connected to Sigil. Severed from the weave, Star Elves rely upon psionic abilities and gifts from a being known as Torama, the 'ancient star'. Star Elves are mostly unable to use magic themselves due to being severed from the weave as part of their banishment, but when it comes to arcane technology, they are second to none. From their perch, seeing their world destroyed from their isolation and slowly encouraged by their aberrant patron towards it, the star elves engaged in an ambitious project - the recreation of the entire multiverse from the energy of the old. The restoration of 'divinity'. And ultimately, the reformation of the multiverse into one where they would be placed as rulers, using the data collected from their 'experiments' as proofs of concepts.

In truth, the world of Toril (and most other material planes) as it is commonly by people in the 'prime world' was brought into existence by an entity controlled by a Star Elven experiment - an entity known as Ao. Ao pulls energy from the 'other world' and feeds it into the 'new reality', all of the dimensions connected to Sigil, in order to feed the experiment's ever-increasing needs, to create new divine entities, and to ensure that those entities are fed the energy that they need in order to perform their duties.

Torama ultimately controls and directs the minds of most Star Elves, and while many believe that they have free will, their people are ultimately its tools for the takeover and recreation of a multiverse with itself at the center - a far realm like horror-verse where it controls all of life in a 'utopia' where everyone will live in eternal bliss and provide it with worship and energy. Only a few exceptionally willful Star Elves have managed to break free of its control, and those who do live lives of fear unless they manage to escape Sildeyuir.

Worlds Beyond Toril
While the Star Elves and Torama are reconstructing a reality based on the old, there exists a number of worlds in the 'old world' that have been explored by adventurers from Sigil. These include:

The Outlands
The Outlands are an outer plane of existence within the larger cosmology. Known primarily for being a planar graveyard, the Outlands are where material planes 'go to die' - bits and pieces of various destroyed material planes wind up here as they are rent asunder by the forces pulling them apart. Devoid of land but temperate and eternally bright, the plane lacks gravity outside of its individual bits of material plane.

Sigil, the Cradle of Salvation, exists within the Outlands, constantly searching for newly destroyed planes at the hands of its leader, the Lady. One of the few permanent features of this otherwise endless void, Sigil is the last cradle of hope for many in the multiverse otherwise without places to call home. As of late, Sigil has been collecting the final remnants of the destroyed material plane of Eberron.