The Dance of the Worlds

“We are the centuries... We have your eoliths and your mesoliths and your neoliths. We have your Babylons and your Pompeiis, your Caesars and your chromium-plated (vital-ingredient impregnated) artifacts. We have your bloody hatchets and your Hiroshimas.
We march in spite of Hell, we do – Atrophy, Entropy, and Proteus vulgaris, telling bawdy jokes about a farm girl name of Eve and a traveling salesman called Lucifer. We bury your dead and their reputations. We bury you.
We are the centuries. Be born then, gasp wind, screech at the surgeon’s slap, seek manhood, taste a little godhood, feel pain, give birth, struggle a little while, succumb: (Dying, leave quietly by the rear exit, please.)
Generation, regeneration, again, again, as in a ritual, with blood-stained vestments and nail-torn hands, children of Merlin, chasing a gleam. Children, too, of Eve, forever building Edens – and kicking them apart in berserk fury because somehow it isn’t the same.”

— Walter M. Miller Jr., A Canticle for Leibowitz

The Dance Of The Worlds, also simply called 'The Dance" is a fundamental mechanism in The Brass Realms, governing the pattern of creation, existence and destruction of the realms.
It is an orderly pattern that is vital to keeping the Brass Realms in working order.

The Oblivion War

The brass realms were established from an unending sea of chaos by the founding Triumvirate.
Limbo and the Fortresses within was erected as a wall between the seas of chaos and the brass realms. In order to fuel the unending war-effort, an influx of energy is needed. As such the forces of Mount Celestia and Hades doneate a fraction of their respective power to create more mechanoids.
The forces of good and evil respectively replenish their forces from the souls of the mortal world, and this is the main reason for its existence; a crucible for mortal souls to be created, collect experiences and ultimately enter the spirit world.

For life, some amount of chaos is needed. However, too much chaos is intolerable to the forces of Law. This is why the dance is designed to contain chaotic influences as well as removing worlds that are destabilized and replace them with new worlds.

The Dance

Creation

Worlds in the Brass Realms are created inside the Divine Machine either by the machine itself or by a deity like Vulcan.

After its creation, the newly made world is then inserted into the Astral Sea.
At This stage, it will be imbued by a large amount of energy by its creator(s) and its development will be closely watched and guided.

Plants and animals
In order to populate the world, the gods will often grab plants and animals from nearby worlds and adjust them as needed. This is why you can often find common animals no matter what world you are on. The Divine Machine and the great Library of Xaaris also contain vast libraries of templates that the creation deities can use to 'cheat'.

Existence

A newly created world slowly drifts away from the center of the multiverse as it takes more concrete Form. The gods wil likely seed it with various animals, plants and perhaps even a sentient species.

The world can remain for eons in this state, with life flourishing on it, undergoing all the trials and tribulations of mortal life.
During its existence, the forces of chaos might influence the world more and more towards chaos. This might be an outright invasion through a hole in the crystal sphere, or a subtle influencing of society or spreading of chaotic energies.

Destruction

If any world in the Brass realms approaches a 'critical mass' of chaos and entropy, it becomes more and more unsustainable.
Things like the laws of physics or magic changing, the world shaking and tearing itself apart or portals into the astral or spirit world opening and spilling forth Spirits & Elementals are signs that the world is approaching the breaking point.

This culminates as finally the barriers separating the realm from the Astral Sea break down and the entire world violently fractures, spilling all its contents onto the astral sea's currents. These floating continents, creatures and other items are then swept 'downstream' until they fall into Sedulon where they are inevitably ground into dust and destroyed, their energy reclaimed.
Worlds in the Brass realms are comparatively short lived, most only lasting a few thousand years. Material Realms that last hundreds of thousands of years are rare.
Spirit Realms last much longer, enduring past the death of every member of a species and many endure seemingly forever.

The Lifecycle of Mortal Species

“Listen, are we helpless? Are we doomed to do it again and again and again? Have we no choice but to play the Phoenix in an unending sequence of rise and fall?
Assyria, Babylon, Egypt, Greece, Carthage, Rome, the Empires of Charlemagne and the Turk: Ground to dust and plowed with salt.
Spain, France, Britain, America—burned into the oblivion of the centuries. And again and again and again. Are we doomed to it, Lord, chained to the pendulum of our own mad clockwork, helpless to halt its swing? This time, it will swing us clean to oblivion.”

— Walter M. Miller Jr., A Canticle for Leibowitz

Mortal species are created, often from a template originally derived from the Firstborn. They are created either by Vulcan or by some other deity.

Mortal species in the Brass Realms generally go through the same lifecycle:

1. Primal existence: Mortal species start out in some form of tribal or animalistic existence. small cmmunities that are technologically and socially simplistic. They are not able to use magic and are struggling with survival, competing with local predators.
2. Civilization: If a mortal species is able to endure and perhaps even thrive against its environment, they will naturally create their first ancient heroes and demigods. They will begin exploring the natural forces and inevitably discover the many forms of Magic. They will organize their society, develop Law and Order and their numbers will increase.
3. Exploration and Colonization: Inevitably a mortal species will begin exploring other worlds and settling empty worlds. This is often to import exotic resources or to seek refuge from some greater catastrophy. These colonies are often small and mostly self-sustaining. Still it allows the species to import large amounts of Riftstone and thus power their arcane infrastructure.
4. Greatness: After signifficant magical research and cultural and economic development, the mortal species approaches its high-point. Armies clad in pristine equipment, knowledgable scholars, prosperity and safety for its citisens. At the high-point of their existence, a species can shrug off apocalyptic events and keep going, crush its enemies and secure its existence for many centuries.
5. Decadence & Decay: After long periods of prosperity and strength, a mortal species will inevitably become decadent. Overly reliant on their ancestor's achievment, used to a life of comfort and luxury, never faced with true danger. These societies will keep going on sheer inertia, but will inevitably collapse. They look inwards and fall into infighting as the species is unable to maintain its technological and cultural institutions. It likely that the species will be wiped out by a competitor or some catastrophy that would have been easily weathered at the species' high-point.

A Species can go through this cycle multiple times, such as surviving multiple multiple cultural downfalls and never quite being wiped out, however it is rare.
There are several species in the Brass realms across the several stages. The Jotnar are past their downfall and their remains are barely clinging on. Their eradication is all but certain. The Tu'vok are at their high-point, but decadence has already begun to set in. Humans have not yet reached their highpoint and were pushed back somewhat by The War of Burning Skies. Meanwhile the Lizardmen are in their cultural infancy, struggling with basic concepts such as law and order and religion, as well as being very low in numbers.