If Clan Pestilens is a religious sect, then Clan Fester is their equivalent to the Templars. Plagues coat each warrior of this band, but this does have the nasty side effect of having the clan-members' bodies literally rotting away. Visible bones and muscle sinew are common, as are blisters along their warriors' tails. Patchy fur is likewise a problem. To put it bluntly, this clan should be given a wide berth.
Clan Fester has the dubious honor of being the designated bad guys in the gristle-mill that is The End Times: Vermintide. With the announcement that Vermintide is now canon and the subsequent lore-book update much more information has been released regarding these unfortunate rat bastards.
1000 years ago, the golden age of the Skaven dawned when Pestilens concocted the Black Plague, leading to an orgy of invasion and feasting, as the Skaven busied themselves by
wiping out a good three quraters of the Empire with a siege of horrible diseases within the cities, dragging sick and weakened citizens into their tunnels, and eating/raiding quickly deserted towns. *FWIP* HERESY!!! This was a case of mild bronchitis exaggerated by legend and the Skaven don't exist.
During this time, Clan Fester was very well respected among the ratmen, even having weaseled its way to a position on the Council of 13, confiding and scheming at the table of the Great Horned Rat itself. Life was good during this time. Over the next millenium, Fester had suffered one problem after another, and the once powerful clan faded into utter obscurity, becoming a Slave Clan to Pestilens. Already diseased as a result of associating with the Plague Monks, their clan recieved a devistating blow when their Broodmothers recently became infected with "Brood Blight", making them infertile. For a race that produces 1d10 pups every week and each mother produces an estimated 1200 warriors and 50 new broodmothers a year each (according to fluff), to call this a massive blow would be an understatement.
Pestilens reaction to this horrific disaster was to take disease samples, document the sickness, and then disassociate with Fester and leave them to die.
With their only prospects throwing their clanrats at enemy clans to act as biological suicide-bombers, or otherwise waste away, the Grey Seer Rasknitt appears, offering them their salvation, but only if they take Ubersreik for him, yes-yes.
As the Stromdorf DLC hints in the Reaching Out mission (with a boss skaven dressed in green and a few Nurgle-symbol motifs around the room), and Vermintide II reveals, Clan Fester struck a deal with Nurgle worshippers (specifically, the Rotbloods clan) to ally with one another, while Rasknitt retained control over the clan by blaming his defeat on the former Warlord (who was too dead to disagree).
Unfortunately however, they and their Norscan chaos ally were out done by a team of five (or four, it doesn't matter) heroes of Ubersreik, where the said heroes sabotaged most of their operation and killed all their leader (Burblespue Halescourge, Skarrik Spinemanglr, Bödvarr Ribspreader and Rasknitt himself). After such a disastrous event, it is safe to said that the clan is either wiped out or stuck in infighting for the rest of the end times.