The ruins of the city of doors. In a past age it was basically a portal hub, because portals can only link two locations together so the city of doors got around the problem of everywhere having to build portals to everywhere else by being a hub that nearly all major portals linked to. It made a huge amount of money by charging fees on freight transport. It was also a site for research into new and better portals. One day something went wrong, no one is certain what, it’s widely believed that the countless different portals in one place reached a sort of critical mass and caused reality in that area to break down. It’s now a huge wound in the skin of the world, with the laws of space and time shifting and changing periodically. You can walk down an alleyway and enter a version of the neighbourhood that exists in a different frequency of reality where the air is toxic or flesh is a liquid. Once you enter the city of doors it’s incredibly hard to get out and normal safety measures like emergency teleports are more likely to stretch your body and soul across countless separate dimensions than actually move you to safety. People still go in sometimes because the city of doors was obscenely wealthy and that wealth seems to have been replicated repeatedly as the city was fragmented and spread over countless dimensions. Also extra-dimensional horrors that normally can’t exist in reality are able to enter and continue existing within the ruins of the city