In the
Wheel of Time series by
Robert Jordan, any of the men gathered and trained to
channel by
Mazrim Taim under order of the
Dragon Reborn.
The Asha'man live and train in what has been dubbed the Black Tower. They have three ranks: Soldier (lowest), Dedicated (intermediary), and Asha'man (highest save for Taim and al'Thor).
Unfortunately, due to the Dark One's taint on saidin, many of the Asha'man quickly went insane and had to be killed. However, if Rand al'Thor and Nynaeve al'Meara were as successful as it seemed in Winter's Heart, that taint may be gone, and the threat of madness with it.