White Frost

White Frost, is one of the omens prophesied by the Winter court as heralding the beginning of the end of the world. Many attributed climate changes to the prophecy.

Hazard

Between the lush and verdant lands of the Plane of Faerie exists an ever-growing blizzard, an unstoppable end to this world known as the White Frost. Violent wind sweep through those lands, the cold of these regions seeping on the essences of living creatures. The frozen nature of those lands affects creatures traveling overland or flying over them. Creatures traveling through such regions must succeed on a DC 14 Constitution saving throw every hour of travel. On a failure, they gain one level of exhaustion from the draining power of the cold winds that permeate the land. Any creature that takes a short rest in this region automatically gains one level of exhaustion as well and taking a long rest results in an agonizing death that only a wish spell can restore life from.

Prophecy

The prophecy of the white frost goes as follows:

The world will open, and many will join,
The fifth element will flow, providing for the world,
Immortals and mortals will fall, taken by death's embrace,
The fey will be white, covered by a robe of ice.
In snow and ice shall he clothe the people,
The sun won't set nor rise, as the shadow will take it,
The world will die, only to be reborn with the new sun.

— Prophecy of the White Frost

Detailed explanation:

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The world will open, and many will join,

This may refer to the War of the Ancients, and how fey and demon crossed portals to Toriel.

The fifth element will flow, providing for the world,

This may refer to the Weave, the fifth element, and to the Nexus Overflow.

Immortals and mortals will fall, taken by death's embrace,

This may refer to the Great Extinction, and to the Draco-Giant War where timeless dragons went extinct.

The fey will be white, covered by a robe of ice.
In snow and ice shall he clothe the people,
The sun won't set nor rise, as the shadow will take it,
The world will die, only to be reborn with the new sun.

However, countless scrolls and books refer to additional prophecy or forgotten parts of this one that talks about heroes that could prevent the white frost. The second prophecy, despite his study by scholar, is much more enigmatic, and goes as follow:

Born once more as he was born before, and shall be born again without end.
The Bronze dragon will be waiting,
He shall break the world again by his coming.
Tearing apart all binds of time.
Yet shall he confront the Shadow at the Last Battle,
And his blood shall give us the light.
Let tears flow, O ye people of the world.

Detailed explanation:

Show spoiler

Born once more as he was born before, and shall be born again without end.
The Bronze dragon will be waiting,

This may refer to Nozdormu,the bronze Dragon god of Time.

He shall break the world again by his coming.
Tearing apart all binds of time.

This reference to time is what led scholars to believe the bronze dragon is Nozdormu.

Yet shall he confront the Shadow at the Last Battle,

The shadow may refer to the white frost, or whatever is causing it.

And his blood shall give us the light.

This may refer to the death of the heroes to save the world or the grievous wounds of the heroes.

Let tears flow, O ye people of the world.

History

The White Frost is a phenomenon that results in the freezing of planes. While the most powerful magics may seemingly slow its pace on a regional level, it is widely understood to be an inevitable end of all worlds. Whether this is the result of a natural force, a malevolent entity, or an entirely different cause is not revealed.
Jörmungandr is the name of an ancient mythical creature of the northern mythology, mostly referenced by northalanders and frost elves. Jörmungandr was believed to be a serpent so large that it wrapped itself around the world, gaining the title of world serpent, and plunging the world into an ice age. Those legend are the foundations of the white frost prophecy, mainly believed by Aen elle (also called frost elves).