— Moiraine Damodred
Mashadar is a malicious, though unintelligent, entity that lurks within the depths of Shadar Logoth. Mashadar was formed during the Trolloc Wars from the xenophobic hatred and suspicion exhibited by the citizens of Shadar Logoth (then called Aridhol). Over time, it grew until it consumed the city and its people. At present, it takes the form of a silvery mist that wanders the streets at night and hides in the ground during the day. The scope of Mashadar is unclear; though it is not omnipresent, it is "vast as Shadar Logoth," and can manifest itself in many locations at once. The slightest touch of Mashadar is fatal, causing a slow and agonizing death. Though it will attack any entity, Mashadar's prey of choice is Shadowspawn, which have avoided Shadar Logoth as much as possible since an entire Trolloc army vanished within its walls. Mashadar is a formidable enemy to a channeller, as it is very nearly impossible to destroy, and will bring its full force to bear on anyone who attacks it with the One Power. It is susceptible to balefire, and will flee when confronted with it.
It is unclear what degree of sentience Mashadar possesses. While on the one hand it is able to select its prey (attacking Shadowspawn with great precision), it is also described as "utterly mindless" and at one time is unable to detect a human mere feet from one of its tendrils.
Though a terrifying force within Shadar Logoth, Mashadar is bound to the city and can not leave its walls. As such, it is believed to be destroyed along with the city during the cleansing of saidin.
The man responsible for the creation of Mashadar is Mordeth, a former advisor to the last king of Aridhol during the Trolloc Wars. His opinion was that the atrocities committed by the forces of the Shadow could only be combated effectively by the display of even greater cold-heartedness and cruelty. As such, Aridhol drew in on itself, considering all outsiders enemies. Towards the end of Aridhol, its soldiers were feared as much as those of the Shadow.
However, Mordeth did not die with the citizens he had led to ruin. His spirit was trapped in Shadar Logoth, unable to leave the walls. His only hope of escape was to accompany a living person to the borders of Mashadar's power, where he could consume the person's soul and, inhabiting the vacant body, leave to wreak his destruction on the world again. He saw his chance in Padan Fain, but the Darkfriend was already so twisted by the Shadow that, instead of displacing his soul, the two merged. The resulting entity has the insanity of Padan Fain, the dark powers of Mordeth, and the memories of both. Unless the two are destroyed, the evil of Shadar Logoth may still be released on the world.
With Mordeth's "escape" from Shadar Logoth, comes Mashadar's as well. One of Padan Fain's abilities is the ability to generate and control Mashadar "tendrils" around himself.
Rand al'Thor and Nynaeve al'Meara destroyed Shadar Logoth and it is assumed to have also destroyed Mashadar. This is due to the Dark One's Taint being siphoned into the area of Shadar Logoth and left the area as a perfect hemisphere.
Memory of Light
Padan Fain with Mashadar escaped from the destruction of Shadar Logoth and attempted to gain vast power by killing Rand al'Thor during The Last Battle. Fortunately Mat Cauthon was drawn to Rand and intercepted Fain. Since Mat was immune to Mashadar, having recovered from its infection earlier, he was able to feign being overcome by it, and kill Fain, stopping his plan of world domination abruptly.