To explain the evil that is mashadar its easiest to consider what created it.
Roughly two hundred years after the Breaking of the World the Compact of the Ten Nations was formed. Society flourished in the hopes of regaining the wonders of the Age of Legends, and one of the nations in this compact was Aridhol. For hundreds of years the Compact held, and these nations flourished, until the Trollocs came howling out of the Blight at around 1000 AB.
A man named Mordeth came to Aridhol at this time, gaining the ear of King Balwen Mayel. The king thought perhaps Mordeth had the solution, use the tactics of the Shadow against the Shadow. The people of Aridhol hardened and became isolated. It became a place to be avoided by even their former allies. The people of Aridhol abandoned the Light, their hatred and suspicion spawning the evil that is Mashadar.
As Balwen withered, Mordeth became the true power in Aridhol. Prince Caar, the son of King Thorin of Manetheren, came to try and win Aridhol back to the Compact. Mordeth had Caar imprisoned as a Darkfriend, but he escaped and fled. When an army from Manetheren came to avenge Caar, they found nothing living within the city. The evil the folk of Aridhol had created had destroyed them. From then on the city would be called Shadar Logath.
Mashadar is some sort of physical manifestation of the evil nature of the city. No enemy had come to Aridhol but Aridhol itself. Suspicion and hate had given birth to something that fed on that which created it, something locked in the bedrock on which the city stood.
Physically, Mashadar is a slightly glowing fog. It moves through the city as aimlessly as a worm burrows through the earth, its touch bringing death. It is not sentient. As a hint of how strong the evil of Mashadar is, Moraine believed that the whole of the White Tower working together could not kill or neutralize the evil of Mashadar in Shadar Logath. Even Trollocs will not enter the city without Myrddraal to force them, and something fierce to drive the Myrddraal.
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