After the fall of Sescheron and subsequent destruction of Arreat, those few Barbarians who survived were struck with madness and despair. Some of them reverted to cannibalism and violence, and were dubbed the Unclean by those who retained their sanity.
The Unclean live to this day, hunting in the ruins of Sescheron, killing any outsider on sight. Years of isolation and constant murder made them fearsome warriors, such that even demons and beasts of the surrounding lands do not dare attack them.
Most Unclean were slain by Skular and other survivors in the years following the city's destruction, but the remaining ones proved too powerful even for them to wipe out. Other Barbarians state that as long as a single one of the Unclean lives, the whole nation is in shame.
"These cannibals are obviously driven by black acrimonious humours."— Abd al-Hazir and Skular(src)
"No. They have fallen to dark magics. There is no other way."
The Unclean, apparently, have some gruesome traditions that make even other Barbarians feel disgust. Painting their faces and clothes with blood, the Unclean often adorn themselves with human skulls, ears, tongues and other body parts. They breed war wolves and have at least some hierarchy left: that is, there are designated Elders (at least one) and at least one king, which they buried in a barrow.
Players encounter the most fearsome warriors of the Unclean tribe in Ruins of Sescheron. Their sheer strength allows them to best all but toughest heroes in combat, so that even other monsters do not attack them.
Apparently, the Wrath of the Wastes armor set was created by the Unclean.