Lankhmar: City of Adventure for AD&D2e. Crossed with Ravenloft - the mists around the city would lead into, and back out of, Ravenloft (yes I know, you're supposed to be stuck in Ravenloft if you're ever unlucky enough to go there, this was a way to mix it up).
...doomed forgotten realms offers a pretty cool premise: what if all of fifth-edition's flagship adventures failed?..it's essentially an apocalyptic campaign set in that aftermath...