"But a tiny black fleck, just large enough to see, floated straight across one of those blue eyes and then across the other in the same line. This Moridin had tapped into the True Power, and more than once. Much more. ...she had not expected the Great Lord to allow one that particular honor. An honor with a bite, as any of the Chosen knew. In the long run, the True Power was far more addictive than the One Power...she did not believe the will existed strong enough to resist the True Power, not once the saa appeared in your eyes. The final price was different, but no less terrible."
Saa, a word drawn from the Old Tongue, is a visible indication of a person who has channeled too much of the True Power, the dark power drawn directly from the Dark One himself. It is one of the side effects from channeling with the Dark One's energies. It manifests as black flecks traveling across the eyes, increasing in number as the person channels more and more of the True Power.
"A black fleck floated across Moridin's blue eyes, then another, in a steady stream. The man must have been using the True Power exclusively since he last saw him to gain so many saa so quickly."
Demandred watches Moridin waste the True Power by destroying a rat during a meeting amongst the Chosen when they discuss Rand's attempt to cleanse saidin