Skavenblight

Skavenblight would be the "capital city" of the Great Underground Empire, at least if it were possible for Skaven to agree on something as important as a captial city. The best way to describe it is a giant pile of rats that explode (both the rats and the pile). It is located north of Estalia and north-east of Tilea, or where Andorra would be in the Old World.

Geologically, Skavenblight resides in the middle of a fetid swamp filled with dangerous wildlife, which fill the swamp with dead Skaven, which fertilize plants which produce black rice like grain for the rats to eat, perpetuating a gruesome cycle of borderline cannibalism that represents their society unnervingly well (borderline? Skaven are fine with and regularly practice actual cannibalism). Skavenblight proper is a ramshackle rat's nest of factories, slums, warpstone generators and narrow mines laid out in no particular order with little forward thinking as to how these structures will interact with one another. These teetering buildings of rusted metal, mud, and rotted wood are held together by the flimsiest of rope and are perhaps as big a danger as the deranged ratmen that reside in them. The labyrinthine layout combined with the unorthodox and unstable architecture make Skavenblight nigh impenetrable to foreign adversaries, further proof that the only enemy truly capable of dealing a decisive blow to Skaven are the Skaven.

Skaven would later survive End Times by teleporting Skavenblight to another dimension. When Horned Rat became Great Horned Rat he immediately drew Skavenblight into the Warp and created more of his Daemons. Skaven had a golden age where there was Warpstone everywhere to be found, they had the direct blessing of the Great Horned Rat, and unlimited space and potential around them. They promptly did the impossible and somehow dug so deep that part of the Warp collapsed into Skavenblight which collapsed into the material realm which is now made up of eight "nearly infinite" planes made of the former Winds of Magic. Skaven now have access to all of reality at once, and can appear everywhere from beneath Sigmar's throne to beneath Khorne's throne. As can be expected the tunnels are not stable and thus only the Skaven are willing to use them usually, as even immortal and deathless Daemons can somehow vanish into the space between spaces never to be seen again when Skaven are involved.