Samma N'Sei, meaning "Eye Blinders" in the Old Tongue, are male Aiel channelers and descendants who serve the Dark One. They dwell in a town constructed by prison labor in the Great Blight, near Shayol Ghul.
The Samma N'Sei are a corrupted and subverted group of Aiel warriors. Aiel men who discover they can channel tainted saidin will go north into the Blight to fight the Shadow. It appears that since the Breaking, many have been captured over time and turned to the Shadow. Afterwards, these Aiel men have been able to channel saidin with the Dark One's protection from the taint.
In TWoTC it is stated that not all of them are able to channel, probably the descendants of turned Aiel channelers, but nevertheless they are trained to become powerful warriors in the Aiel way.
There are inverse differences among the Samma N'Sei to Aiel culture. They veil in red instead than in black. Proper Aiel veil themselves before killing, while the Samma N'Sei unveil before making a kill. They also file their teeth to points.
In TWoTC it is stated that Samma N'Sei were groomed as a surprise weapon in the Last Battle, nevertheless some of them had an earlier mission to find and destroy the Eye of the World before the Dragon Reborn could reach it, but their mission failed. Anyway it can be guessed that they took their name from this mission.
During the Last Battle in Thakan'dar the Samma N'Sei took by surprise the forces of the Light, consequently the Aiel Wise Ones took particular responsibility to fight them as a 'toh towards their allies.