Lore:The Blight

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The greatest, and most obscure, threat is the Blight, a mysterious weather-like phenomenon emanating from the crater of Dagoth Ur, warping and poisoning creatures in its path, and creating diseased horrors that attack travelers and outlying settlements. — A Savant

Ash storms carry the Blight far beyond Red Moutain

In 3E 427, the ruling powers of Morrowind, already weakened by questions of authority, were further threatened by the re-awakening of the Blight. This ancient curse is carried from Red Mountain beyond the protection of the Ghostfence through ash storms. It is believed to be created by Dagoth Ur as a means to spread his influence, softening defenses and tying up resources of the Tribunal Temple. House Redoran is particular strained in its territories and maintains great volunteer forces in Maar Gan and Molag Mar.

Blight diseases resist regular Cure Common Disease, and are of two kinds: wasting diseases which attack one or more of an organism's systems, and abnormal growth diseases, which distort the organism's functions and structures. Natives avoid exposure to the Blight, and wear special protective garments when traveling in regions exposed to ash storms. Those infected can be cured by altars, potions and spells.

Ash storms even reach the cities

The Blight also affects creatures, making them stronger and more dangerous to unwary travelers. It affects the profits drawn from the land, leading to closures of mines, especially egg mines. As the infections grew, the Empire laid down an embargo on wares from Vvardenfell, leading to some starvation and further isolation of the Dunmer on the island.

But most serious are the plagues and diseased hosts produced by the blight storms sweeping out from Red Mountain. Vvardenfell and all Morrowind have long been menaced by the legendary evils of Dagoth Ur and his ash vampire kin dwelling beneath Red Mountain. For centuries the Temple has contained this threat within the Ghostfence. But recently the Temple's resources and will have faltered, and the threat from Red Mountain has grown in scale and intensity. If the Ghostfence should fail, and hosts of blighted monsters were to spill out across Vvardenfell's towns and villages, the Empire might have no choice but to evacuate Vvardenfell district and abandon it to disease and corruption. — A Short History of Morrowind

The Velothi Mountains are a natural barrier, protecting the rest of Tamriel from the Blight.