Planar Collapse

Even outside the Plane it reigns on, a primal elemental has a certain reputation. Each plane has a fabled entity ruling over it, and it is from these entities that the end of the world will come. Perhaps the tales of the end of life were once not so centred on the planes, but after the First Isolation and its cataclysmic consequences, this is how it eventually got prophesied.

One day, the world will come to an end. One day, our plane will stop to exist. One day, the planes will collapse. Of course.
Indeed, the Isolation¹ gave us all a taste of what happens when the planes get disturbed, but even that was but a scratch, a meaningless bruise, in the face of what is to come, one day. Perhaps it would be wise to believe the Isolation preferable to the repercussions of their restlessness. Now, the Monarchs are calm, undisturbed, but one day, they shall stir in their rest, one day, they will come awake once more, and that day, they will not be giving water for the crops or fire for the forge. That day, they will bring about destruction.
That day, the world will end.
The veil between the planes will bend, crack, tear, shatter. The Elemental planes will bleed into each other in a life-ending collapse, the Monarchs will enter a terrible dance, and they will bring over our end, the end of the material plane and its reflections. There will be nowhere to run.
Arythlas will take the place of the sun and set all under its wing ablaze. Dessak will tear the earth open, collapsing it under its own weight. Desyvn will bring chaos to the seas, rising terrors from the depths. And Erysth will rend the sky open, opening the way to storms to spread the destruction to every last parcel of world.
One day.

— Transcription of the oral tale as recited by a bard

1. The term First Isolation was not needed when there had only been one Isolation, prior to the recent events, thus the use of the term "the Isolation" in this document.

The Elemental Monarchs

Never actually seen, the existence of these beings is however never doubted, as their powers can be felt in other ways. Much like deities, though they are not generally considered to be such, they are unseen forces to be reckoned with. Thankfully, every tale and accounts affirm and reaffirm that these beings remain mostly dormant in their domain, as their full awakening is feared is feared beyond measure, as can attest the tale above.

Arythlas, the Phoenix

Majestic, the sight of the Phoenix instills respectful, terrified awe. So bright, it is said your eyes would melt while its flaming silhouette would imprint in your mind.

Dessak, the Earthshaker

The mearest movement from this great landmass would shake the ground to the core, causing earthquakes and erruptions, while the shear weight of the Earthshaker would crush its witnesses soul into a million pieces.

Desyvn, the Leviathan

A veritable "monster of the depths", the Leviathan is the greatest fear of sailors since the beginning of time. If the world-ending serpent were to surface, however, it is everyone who should fear the tidal waves rising in its wake.

Erysth, the Living Tempest

With the appearance that got it its title of Living Tempest, the Monarch of Air is already fearsome in its depictions. Raging thunder, lightning, furious winds, and like the other monarchs, unfathomable size all make for a fearsome vision.