Gnawholes are the ragged, reality-chewing tunnels through which the Skaven travel between and within the Mortal Realms, the signature sorcery of the ratmen's teeming empire. Bored through the fabric of existence itself by a blasphemous blend of warp-science and dark magic, a gnawhole can open almost anywhere, disgorging a verminous horde in the heart of a city that believed itself secure.
Unlike the stable realmgates that stitch the planes together, gnawholes are unnatural wounds in the world, torn open by the Skaven and often collapsing behind them as quickly as they appeared. They reek of corruption and warp-stuff, and their passages writhe through a nightmarish between-space that only the ratmen seem able to navigate. This grants the Skaven a strategic mobility unmatched by any other people, letting them strike, plunder, and vanish before a defence can be mustered.
The existence of the gnawholes is a large part of why the Skaven are so impossible to eradicate. Defeated in one realm, they simply gnaw their way to another; walled out of one kingdom, they erupt from beneath its foundations. Their vast underworld is said to link every gnawhole into a single continent-spanning web of tunnels, a hidden empire beneath the skin of reality from which the ratmen can menace all of creation at once.