Clan Mawrl was a Skaven Warlord Clan that once inhabited Under-Altdorf. During the Second Skaven Civil War, it swore allegiance to the Clan Pestilens, one of the Great Clans, as part of the alliance of clans known as the Pestilent Brotherhood and had received a Pestilens-created artefact known as the Wormstone as a reward for its loyalty in that conflict. After its members were infected by a mortal disease caused by their possession of that artefact, all of Clan Mawrl, from the lowest Skavenslave to its Clan Warlord himself, was buried alive in a massive grave by the other clans who inhabited Under-Altdorf, completely wiping out the clan.[1b]
This was not the typical act of malice common among the Skaven clans, but an act of self-preservation. The fell power of the Wormstone had not proved a boon. The artefact was an alchemically-altered chunk of Warpstone that when touched would infect the unlucky individual with a plague of yellow-glowing maggots that would quickly extinguish their life in horrifying pain. This infection swiftly proved to be Clan Mawrl's doom, infecting and killing every Skaven in the clan.[1b]
In order to make sure this infection died with Mawrl, the remaining Skaven clans in Under-Altdorf caused their warren to cave-in, burying them alive and the Wormstone deep. The Wormstone, and the bones of what was once Clan Mawrl, would not be unearthed for many years. [1a]
The Rat-Beast discovered alongside the Wormstone when it was later unearthed years later from Clan Mawrl's mass grave by Grey Seer Skabritt who sought to claim its power, may have been one or several Clan Mawrl Skaven mutated together during their long confinement, as it was said to be rat-like and had fed on the unnatural maggot-infestation wrought by the Wormstone.[1a]