Alright this is my very first time posting a theory so here it goes...
I have often thought of the evil that was present in Shadar Logoth. Also the presence that is Machin Shin(The Black Wind) that exists within the ways. What I bring to you today is a theory that they share a commonality that maybe people haven't yet thought of. I will present evidence to this effect and allow you to draw your own conclusions based on this evidence.
Lets start with listing the facts of Mashadar within Shadar Logoth.
We all know that Mashadar was the entity created by the people of Aridhol because of their strict desire to destroy the shadow they themselves began to enlist the methods of the Shadow and worse in order to fight it. People in the world often said "Better to encounter a Trolloc than a Aridhol soldier".
Aridhol eventually destroyed itself. Mashadar was created in place of that evil with the soul of Mordeth(The man responsible for driving and leading the people of Aridhol to their destruction.) waiting for some pour soul to devour and possess so he could walk the world again. Mashadar being a grey like fog that devours its victims.
Alright now lets list some facts of Machin Shin(The Black Wind).
It inhabits the ways(which contain no wind at all) It creeps like a black fog upon its victims and kills them, drives them insane or leaves them a soulless embodiment of what they were before.
It has been described as 1000's of voices speaking of pain and anguish and death and torture and various other inhuman acts.
Some similarities between the two have already been listed here, they both share a fog-like essence and do very similar things to their victims that they catch.
There are some other similarities they share as well. One primary similarity is that they are both regarded as not of the Shadow but of other evil as they will kill shadowspawn as quickly if not more so than any other. Shadowspawn will only go into these areas with the strongest of influence at their backs.
Lets talk time frame.
Aridhol met its fate during the trolloc wars when it destroyed itself.
The ways began to show serious signs of corruption years later during the war of Hundred years...
Aridhol was a Ogier built city which means there was a steading nearby and therefore a Waygate to get to that Steading.
We know for a fact that Shadar Logoth had a waygate because it was where Sammael was last seen standing before Mashadar overtook and killed him at the of ACoS.
And now the biggest piece of evidence is of course Padan Fain.
Fain's soul was touched by the Dark one and therefore when he was consumed by Mashadar, Mordeth wasnot able to completely overcome Fain but instead joined with him creating a Fain/Mordeth entity.
When Fain walked the Ways, Machin Shin came in contact with him he was met with the voices of The Black Wind, some greeted him, others feared him. In any case he was allowed to walk free of the Entity that didn't even leave shadowspawn living.
So after all this rambling about facts here is the theory. Machin Shin is a off shoot of Mashadar. At some point either someone opened the Waygate at Shadar Logoth and Mashadar creeped itself in, or over the course of a long period of time Mashadar was able to seep itself past the waygate. Either way that piece of Mashadar was cut off via the waygate from the rest of Shadar Logoth and unable to leave once it was trapped. Which would explain why it wasnt affected by the cleansing of Saidin. The Ogier obviously would never enter they ways after the corruption was known and even more so would never go to the way gate at Shadar Logoth thereby creating a block in the history of that waygate being recorded at any point during the last 2500 years.
For the theory that the taint created the problem with the ways I refer to a statement a friend of Mine Nate S. made "While the ways were made by men who had the taint, from what we've seen the taint only affected the channeler himself, but it doesn't really effect the chaenneling itself" When ever Rand or the other Male Channelers, both the current ones and the Male Aes Sedai of the past, they created weaves out of insanity but the weaves themselves are not tainted. Rand described the taint as an oily layer of ooze over the top of pure water. The taint was a barrier between men and Saidin. Saidin was still pure on the other side of the taint.
So, Are Mashadar and Machin Shin basicalaly the Same entity?
I leave it to you to decide whether or not this theory holds any weight or validity to the Wheel of Time universe.
Again I want to thank Nate S. for his input on this theory.
Please Let me know what you think.