Towers of Midnight. After noticing the Aelfinn are now after them, Mat and co. decide to try to enter their realm and find their Doorway that exit to Tear. They do find it, but they find it's in pieces. Then they are screwed, the Aelfinn surround them and Mat starts going through every little details of his encounters he can remember and figure out the Ashan'darei can make a portal. I doubt the Aelfinn destroyed their own Portal, but I can sure see why Moridin might have decided to destroy it as he exited Sindhol after killing Cyndane. He knows who Moiraine is, for one thing, and I get the feeling he's figured out the Aelfinn have helped Rand or his friends, and if they did, Rand might send others there to get more, and more, and more. Moridin probably worried Rand might have used the Portal to ask questions, and that he could send other people for more.. he probably blame himself about leaving that Portal (it's unlikely he didn't know it wsd in Tear, the AS know). Moridin is very intelligent, he might have suspected Aelfinn help when he learned Rand planned to cleanse saidin, something Aginor himself had not even dwelled on (and Aginor - not to mention Cyndane - was well placed to know Rand's OP knowledge was, shall we say, fairly limited. It's him, Osan'gar who discovered Rand planned to attempt a cleansing, which he seemingly routinely reported to Shaidar Haran but totally disbelieving Rand would even know where to start. Then it seems Moridin or SH put Cyndane in the wrangler and got out of her stuff she had previously hidden from them. He/they probably questionned her about Rand's OP knowledge, about the science involved too... maybe.. and she at least revealed then to Moridin Rand had access keys to the Choedan Kal. That's when Moridin/the DO called the meeting to order all the Chosen to stop Rand, to kill him if necessary. At that meeting Moridin fondled Cyndane commenting she was not anymore keeping any secret from him.).