Regarded as some of the finest warriors in Skaven society, Clan Skab Clanrats are often leased out to other Clans for service. Even more important, Clan Skab tends to produce more Black Skaven than other Clans, and so they have an inordinately high number of Stormvermin in their ranks. These expertly trained warriors are often sold to other Clans to act as guards for Chieftains and Warlords, or are claimed by the Grey Seers for similar purposes.
Alas, Clan Skab is no more. It was during the 2499 IC (Imperial Calendar) assault on the city of Nuln that Clan Skab was placed under the extremely questionable leadership of Grey Seer Thanquol. The resultant attack saw Clan Skab (along with half of the city) destroyed. Ironically, Clan Skab had been a serious threat to the Council of Thirteen. While Thanquol had no idea of this, he quickly took credit for the demise of such traitors. Admittedly, this means that black Skaven are less common than previously, but the Horned Rat does work in mysterious ways...
As of Age of Sigmar, it seems that the Horned Rat was generous enough to bestow a second chance on Clan Skab; however, as the clan is listed as a Third Echelon within the Clan Pestilens hierarchy, it would suggest that this could be a whole new clan with a whole new (and decidedly more sickness-influenced) lifestyle. Only time shall be able to tell.