Unless stated otherwise, all information herein is taken from AMoL, Prologue.
The Samma N'Sei, which means "Eyeblinders" in the Old Tongue, are male Dreadlords. They live in the Town near Shayol Ghul. They wear clothing that looks like the Aiel, but with red veils. They only remove their veils to kill, and usually only for kills they most eagerly anticipate. Some, but not all, have been Turned to the Shadow. Those who have been Turned have teeth filed to points to mark this.
Some Samma N'Sei are recruited instead of Turned, and undergo some sort of training. It is said that those that do not breed true will often wash out of their training and die. Isam considers them very temperamental and ready to kill on a whim. They often kill just to explore new ways to use their powers.
"Red veils. They kept them up always. Well, almost always. If you saw one lower his veil, it was time to kill him. Because if you didn't, he'd kill you." (A Memory of Light, Prologue)
"The Samma N'Sei, the Eyeblinders, had always been touchy and full of pride. No, touchy was too mild a term. They required no more than whim to take a knife to one of the Talentless. Usually it was one of the servants who paid. Usually." (A Memory of Light, Prologue)