I was wondering if the fog in the rebel encampment that Rand visited while posing as Tomas Trakand is some more of Mashidar from Shadar Logoth? 

I don't think so, it was just a bubble of evil. For one, it seemed to touch everybody, like regular fog. It's just the things that were in it that killed people.

Ahh that makes sense - Fain simply used it to his advantage..I was guessing that since he had the dagger that somehow it had manifested itself within him and thus sprung forth when Rand discovered him; forgot all about the bubbles of evil!

It could be one of the tricks from Mordeths power.

Yeah, I always thought it was one of Mordeth's tricks, its similar to Mashadar.

 

My guess it is a version of the Power Mordeth came across while looking for a way to beat the Shadow at its own game. (See the Unseen Eyes Theory)

 

So like Mashadar was a product of Mordeth's strange discovered power, this was also some form of that Power.

RJ said it was a bubble of evil.