The Ash'abah was an ancient pariah tribe of Redguards that existed in the northern wastes of Hammerfell who were known to have "unclean interactions" with the risen ancestors. They were believed to be supported by The Forebears. The tribe was led by the headman Marimah, who led an ambush on the undead armies of Suturah and destroyed them with forbidden magics.
When the Yokudans first arrived in Tamriel, they began to conquer the elves that lived in Hammerfell. The elves were pushed back slowly but surely until they made their final stand in the Ash'abah Pass in Hammerfell. While victory seemed assured for the Yokudans, the elves realised their enemies had a weakness: their veneration for the dead. Even a zombified Yokudan was considered sacred, and they would raise no weapon against it, even to defend themselves. Knowing this the elves killed the Yokudan king and reanimated him, hoping it would compel the Yokudans to leave.
One of the king's two sons instead slew his undead father and the necromancer that raised him despite the protests of his people. He accepted the exile his brother sadly imposed on him. He founded a tribe of like-minded Yokudans who were willing to dishonor themselves for the good of their people. They became the first Ash'abah.
Decades later, necromancers threatened the Alik'r again, this time with an army of risen dead. The Yokudans would not defend themselves and were being butchered. It was then that the Ash'abah returned from their exile, led by the king's exiled brother, and drove the necromancers back. They then returned to their exile in the Alik'r desert.
So it has been since that day that whenever undead threaten Hammerfell, the people accept the aid of the Ash'abah warriors, albeit grudgingly as the Redguards still view them as dishonorable. In response to the dislike, a common saying among the Ash'abah is "True honor need not be named".