Cursed Marshes

Marienburg, the most infamous trading port in the Old World, lies in the midst of the stinking swampland the Cursed Marshes. It is within these marshes that the Skaven Clan Scruten have their hidden stronghold, from where they launch their insidious raids beneath Marienburg and its surrounds.

Around the year 2000 the clan was led away from Skavenblight by the Grey Seer Kritislik. Deep beneath the marshes of Marienburg he led them, into the ancient and abandoned Skaven stronghold simply known as Rat Rock, to the north-east of the human city. It is said that several Warrior clans once competed for dominance within the keep, and it was once teeming with thousands of Skaven and their slaves.

However, the stronghold was doomed once a new threat became apparent. The rival Warlords united, combining their strengths to fight this menace, yet it was to no avail. Who this foe was has been covered by the shroud of time, though stories tell of some form of daemonic, cyclopean swamp-dwellers. It is said that these ancient daemons were somehow a part of the marsh itself, able to control the mists and command the creatures that dwelt within them. Whoever this enemy was, it is believed that they and the Skaven wiped each other out. Marienburgers fear the swamplands, and venture into them only when necessary as a result. The Skaven of Clan Scruten take advantage of this, and many of their activities are attributed to mythical daemons of the swamp -- whether they ever existed or not.

Source
pg. 41
 * White Dwarf #302