I haven't seen this idea discussed before so I thought I would mention here. Unfortunately, I do not have my books right now so I cannot provide exact quotes.

Key point #1:

The shadar logoth evil is the counter-taint to the Dark One's taint. They are basically inverse waves of each other-- if they are placed together then they cancel each other out. Rand obviously used this to great effect when he devised a way to cleanse saidin of the taint.

Key point #2:

We know that the Dark One has been around forever, but we also have some evidence that the Shadar Logoth evil is very old. At the end of EOTW Aginor said that he tracked Rand's party from Mat's ruby dagger. Aginor called it "an old thing, an old friend, an old enemy." Why did Aginor refer to this dagger as both an old friend and an old enemy?

Key point #3

In TGH Lanfear recognizes the evil of the ruby dagger and thinks that putting it in the case with the Horn of Valere with help protect them.

Key point #4:

At the end of WH Demandred travels to the outskirts of Shadar Logoth. Demandred immediately recognizes what Rand was planning and admits it was a brilliant plan. Demandred obviously has some familiarity with the Shadar Logoth evil and how it opposes the Dark One.

Key point #5:

In EoTW we see Rand can sense the evil/taint of the ruby dagger, much in the same way that he can sense the Dark One's evil/shadowspawn/taint.

Key point #6:

The Age of Legends was a very peaceful society where very little evil was known. The Dark One was still confined to his prison. We do not know about the existence of the Shadar Logoth evil during this time.

Putting it all together :

Since the Age of Legends was a very peaceful time I speculate that the Shadar Logoth evil was either very limited or not present at all. There is very little need for the Shadar Logoth evil when there is no Dark One to oppose.

We do know that several forsaken (Aginor, Lanfear, Demandred, and probably Ishamael) are very well aware of the danger of the Shadar Logoth evil. Thus, I suspect that the Shadar Logoth evil came into prominence sometime around the same time that the Dark One did. Aginor called it an 'old thing' so this evil most likely existed way before the creation of Shadar Logoth.

Building upon everything I have mentioned, I now want to throw out a possible theory. What if the Shadar Logoth evil and the Dark One both escaped at the same time when Lanfear opened a hole in the Dark One's prison? I am just throwing this idea out, and it could be an absurd idea, but I think it generates an interesting discussion.

If the Shadar Logoth evil is the counter-taint to the Dark One's taint, then would it not be the perfect method to keep the Dark One contained? What if the two energies could be wrapped around each other and knotted in such a method that neither could escape without annihilating each other?

What if Lanfear totally disturbed this equilibrium when she drilled into the bore? If this is true, then it means using the Shadar Logoth evil could be essential in re-patching the Dark One's prison.

I am just throwing out a wild idea so that I have something to keep my mind off the release of TGS in 2 weeks. There are definitely problems with this idea. For example, why does the Shadar Logoth evil appear to have more freedom in the world than the Dark One does? If the Dark One is still not fully escaped, then this would mean that the Shadar Logoth evil is not fully escaped either.

Also, if the Shadar Logoth evil helped imprison the Dark One, why would Aginor refer to it as an old friend as well as an old enemy?

Feedback is wanted, bash this theory to the ground if you can