Verin Mathwin gave a letter to Matrim Cauthon before she used Traveling to take the Band of the Red Hand to Caemlyn in Andor. He swore an oath that if he opened it he would carry out its message and would be allowed to leave Caemlyn in ten days. If he didn't he could leave in thirty days and be forced to burn it. He chose not to open it and it was opened by Olver while Mat was away with Thomdril Merrilin and Noal Charin in the Tower of Ghenjei.
If you are opening this, then I am dead. I had planned to return and release you of your oath in a single day. There are many potential complications to my next task, however, and a large chance that I will not survive. I needed to know that I'd left someone behind who could see this work done.
Fortunately, if there's one thing I believe I can rely upon, it is your curiosity. I suspect you lasted a few days before opening this letter, which is long enough for me to have returned if I were going to. Therefore, this task falls upon you.
There is a Waygate in Caemlyn. It is guarded, barricaded , and thought secure. It is not.
An enormous force of Shadowspawn moves through the Ways toward Caemlyn. I do not know when they left exactly, but there should be time to stop them. You must reach the Queen and persuade her to destroy the Waygate. It can be done; walling it up will not suffice. If you cannot destroy it, the Queen must bring all of her forces to bear upon guarding the location.
If you fail in this, I fear Caemlyn will be lost before the month is out.
Verin reveals a huge attack on Caemlyn by Waygates that were supposed to be sealed by Loial and Karldin Manfor. Somehow, the Shadow found a way past the wards. She knew of the Waygate and the attack on Caemlyn due to her infiltration of the Black Ajah. However, she made a very large mistake overestimating Mat's curiosity. She only knew Mat when he was under the influence of the Shadar Logoth dagger. Mat didn't open the letter as planned, instead, far later, Olver opens it when the attack is beginning to take place.