In TSR, ch. 31, we learn that Fain managed to capture a Myrddraal and eventually made some sort of unknown deal with it to control it. The Myrddraal is then briefly mentioned in TSR, ch. 56, where it is stated that "the Halfman feared Isam would find it" (by the way, another early clue that Slayer hunts renegades of the Shadow, like Asmo was one..). Later in LoC, ch.28, we see the Myrddraal again with Fain in Caemlyn, now apparently completely controlled by Fain.

I think since this scene in LoC, ch.28, Fain's pet Myrddraal wasn't even mentioned again, although Fain thinks in LoC that "it could be useful still."

Fain's Myrddraal might e.g. reappear in the anticipated Caemlyn conflict. Maybe Fain will use the Myrddraal, especially his ability to move in the Shadows, to try to capture or kill the pregnant Elayne and thus hurt/lure Rand (though the blunt dagger would likely protect Elayne from the Myrddraal).

Perhaps Fain's Myrddraal - plus the questions exactly how & why Fain controls it and what the Myrddraal is up to now - is even the little detail, which BS said will become important in the next books. Since Fain's Myrddraal seems also connected to Slayer - at least by knowing Isam - the Myrddraal might furthermore play a part in the Shadow's attempts to find and kill Fain.

So, do you think this capturing of the Myrddraal just happened to demonstrate how dangerous Fain's powers are and it won't appear again? Or do you think this Myrddraal will play a certain role yet?