The Ark of the World
Born out of a discussion mostly with !AntiTheist! and Gorinich_Serpent on Phlox's GLOG Discord server.
There is a multiverse - a planar set anchored around a prime material plane, not a collection of timelines - that is falling apart. Magic has torn it apart, and it is now dying - slowly, painfully on some planes, and terribly swiftly on others.
Some of the people of this world have not despaired, however - they partake in a great work, the construction of a massive Ark that can carry the people of their world to a new multiverse, one safe from the destruction that has ravaged their world.
The Roots of the Chaos
Extraordinary Scale
It began with a single spell, developed by the archmage Anamado of Xi'Tam - now one of the most reviled names in history.
Anamado's Great Wall
R: 10 feet D: permanent
You conjure an upright stone wall in an open space that is at least partially within range. The wall is [5 x dice] feet thick, [40 x sum] feet long, and [20 x sum] feet tall. If there is anything obstructing the wall or there is no contiguous surface for the wall to be conjured upon, the spell fails. The wall is affixed to the surface.
The spell was revolutionary. It totally exceeded all prior understandings of magical effect scale as a function of input power. Had it stayed solely the knowledge of Anamado - who was, in truth, a wise man who understood some of the risks the spell presented, though none could foresee the full extent of the dangers - the spell likely would have remained a wonder, a curiosity. However, Anamado's notes where stolen, then copies were sold, then copies were shared, and the principles Anamado had uncovered through extraordinary genius fell in to the hands of the masses. These were initially called the Anamadan Principles, with the metaphysical theories that enabled them being named Anamadan metaphysics.
The First Anamadan Spells
Two years after Anamado's notes were in the hands of every wealthy or well-connected wizard, two spells began spreading:
Anamadan Hill
T: touched point D: [sum] minutes, then permanent
You create a mound of dirt with a height and radius of [30 x sum] feet. It grows from the targeted point over the course of the initial duration, after which the dirt is mundane dirt.
Anamadan Step
T: self and/or up to [dice] touched creatures D: instant R: [20 x dice x sum miles]
All targets are transported to a location known to the caster within range. If the caster does not accurately target a valid location, the spell fails.
Anamado's spell was not a fluke. Anamadan metaphysics could be used to create massively more efficient and powerful spells. Generally (but not always), the effects weren't novel, but they had extraordinary range or scale duration.
Spells of Mass Destruction
The first directly destructive Anamadan spell - created about three years after Anamado's notes were leaked - was, perhaps, the true beginning of the end.
Crack the Sky
R: anywhere exposed to the sky you can clearly and uniquely describe the position of T: a [50 x dice] ft radius D: see text
You identify a position. After [dice]d6 hours, a magical gate will be opened orbitally over the location. Then, over the next [sum] rounds, a glowing beam of ominous humming light shines down from orbit onto the location. Then, for the next [dice] rounds, everything in the area exposed to the sky takes [5 x sum] damage each round as a massive force beam scorches the earth.
Suddenly, even weak wizards could devastate their enemies from a vast range. The only saving grace was the slow onset of the effect - the spell was mostly only effective for cracking fortifications and destroying structures.
The second offensive Anamadan spell was far more deadly.
Anamadan Spark R: 1 mile T: a burst centered on the caster D: instant Everything within range except the caster takes [dice + half sum] fire damage (save for half).
Mad wizards and terrorists could devastate countrysides with little risk (so long as they took precautions against being caught in the ensuing blaze). Thousands upon thousands died from the initial bursts. Wildfires, burning cities, and famines caused by crop-burning killed even more.
Conventional wisdom holds that Anamado took his own life after seeing a major city essentially annihilated by the Spark.
Wizards of Mass Destruction
"The Plural of Wizard Was War"
Quote from The Last Continent by GNU Terry Pratchett
When war - inevitably - broke out, Anamadan magic was unleashed at cataclysmic scale. The war spread from the prime material to every semiphysical plane. Planes less stable than the prime material were collapsed or devastated by planar destabilization magic. Panicking extraterrestrial and extraplanar civilizations were dragged into the war, desperate to contain the devastation. Many gods even fell in the ensuing war when it was discovered that Anamadan principles could be used to efficiently harm divine beings by exploiting the scale of divine power.
Their efforts were nearly in vain. The Anamadan wizards - which were beginning to be called Annihilation Magi - were unimaginably powerful, and fighting fire with fire only lead to greater devastation. However, after almost ninety years of war and chaos - with every faction and group desperately battling for survival - most of the Annihilation Magi were slain and their grimoires burned.
After the war ended, those who survived called it the Trial, and believed they were safe. A few surviving Annihilation Magi continued to vie for dominion or hid themselves in their sanctums, but the multiverse was believed to be on the mend.
A Slow Collapse
The dimensional destabilization magic that destroyed some of the outer planes was thought to be ineffectual on the prime material. However, a few years after the Trial ended, it was determined that it was only much slower on the prime material, and when it collapsed, the entire multiverse would collapse with it.
Attempts to halt the collapse all failed. The only option left was escape.
A Way Out
With doom inevitable, the search for a way out of the multiverse began. However, travel between multiverses - or even locating a viable multiverse to travel to - is far, far more difficult than travel between planes.
Metaphysical Intersection
One of the greatest difficulties with multiversal travel is metaphysical compatibility. Different multiverses can have different thaumaturgical metaphysics (as distinct from ontological metaphysics - though the precise nature of that distinction, or if it is even real, remains a question for the scholars) and different physics. Bringing foreign metaphysics to another multiverse leads to derealization, where the local reality axiomatically erases the existence of the intruder. Consequentially, it is necessary to travel to the secondary multiverse using only metaphysical principles common to both multiverses.
As thaumatic metaphysics can be used to alter physics (given a sufficiently effective set of metaphysical principles), incompatibilities in the laws of physics can be overcome by transforming the physical material sent from one multiverse to another in-transit.
The Ark
The people of the primary multiverse call this intersection set the Ark metaphysics, as they are the metaphysics used in the construction of a great Ark to bring them from their doomed multiverse to a new one, safe from the dangers of Anamadan magic.
The Ark is the work of nations - or at least, what remains of nations after the Trial. Its construction must be performed solely using Ark metaphysics, as the taint of sealed metaphysics could cause critical components to be derealized when attempting to enter the secondary multiverse. The shear amount of matter generated by Anamadan metaphysics (any matter generated by sealed metaphysics will be permanently compromised by it) means that finding the raw materials needed to construct the Ark - which is the size of a city - very difficult and require intense, careful purification processes.
To prevent too much from being lost in a single attack, construction of the Ark is scattered across many locations, with redundancy for many components.
The People of the Ark
In addition to the various peoples of the world united in their desire to survive, the People of the Ark are joined by several notable factions.
The Heavenly Hosts
With the collapse of the afterlife planes inevitable, many of the hosts of heaven have joined in the work, even though their innate magic will prevent them from ever escaping the doomed multiverse. Foremost among them is the Metatron, an arch-solar. In a working of unimaginable complexity, it has reshaped itself to a form that can ferry the souls from the heavens of the primary multiverse to those of the next - though it will die in the execution of its duty.
Lunarians
The devastation of the Trial was spread even to the moon, and the technologically advanced Lunarians, their home destroyed, bring their skill at artifice to the Work. Lunarian emplaced force fields are the last and strongest line of defense around critical, irreplaceable components of the Ark.
Gold Dragons
Though most dragons (all of which are too innately magical to escape) have succumbed to ennui, denial, or spite, the Gold remain true to their principles, even though they are doomed.
The Court of the Ark
The fey courts were shattered during the Trial. Some - adapted to Ark metaphysics thanks to the chaotic nature of Fey magic - have united to form a new Court to aid the Work, though they remain fey and wild in mind and nature.
The Seekers
Raw materials untainted by sealed metaphysics are invaluable in the construction of the Ark. Seekers are adventurers who locate items and scout locations of untainted natural resources, as well as handle threats to the Work that require smaller teams ranging far afield.
Enemies of the Ark
The Spite of the Sealed
Many of those who cannot escape due to excessive sealed metaphysical taint - including many outsiders (those whose home plains have yet to collapse, or who escaped to the prime material) and dragons - are often spiteful, seeking to destroy the Ark as it cannot save them.
Mad Magi
Powerful spellcasters who heavily employed sealed metaphysics are often also unable to travel to the secondary multiverse. Sometimes out of spite, sometimes out of a mad desire to hold on to what power they can before the universe ends, many of these spellcasters seek to enslave the people of the Ark or simply taint construction with sealed metaphysics - often through massive Anamadan spells.
As the people of the Ark do not use sealed metaphysics, their spellcasters are far less powerful the those who use the full primary metaphysics, especially as Anamadan metaphysics are within the sealed metaphysics.
The Undead Hordes
As a consequence of the application of Anamadan metaphysics to the field of necromancy - and the massive amount of corpses produced by the Trial - the undead haunt the earth in unprecedented numbers. Often, hordes will be drawn to the population centers necessary to support construction of the Ark.
Cataclysmic Conditions
The ravaging of the Trial and the effects of ongoing multiversal collapse lead to devastating weather phenomena and supernatural disasters. In addition to the threat posed by simply destroying progress on the Ark, some of the thaumaturgical disasters also risk tainting the Ark with sealed metaphysics.
Factionalism
Inevitably, even when united by the Work, the People of the Ark remain people - and are subject to factionality and infighting. Longer-lived shepherds, included the heavenly hosts and the handful
Time
Of course, the greatest enemy is time. While it is not possible to precisely calculate the time until collapse, even the highest estimates are less than a thousand years, and the great Ark is the work of generations.