I’m probably going to use this as a starting point for an Apocalypse World game, but with a bit more development it could work as a setting for Blades in the Dark.
“No one would have believed, in the last years of the nineteen century…”
It’s now one year on from the invasion (from H. G. Wells’ ‘War of the Worlds’). The Martians didn’t die out, and the surface world is encrimsoned with the red weed and stalked by Martian tripod war-machines. Most of the human population has either perished or have been herded into camps to serve as food and slave labour for the alien overlords.
Yet some people still remain free, slipping betwixt the cracks like rats. They need to stay quiet, to stay hidden, and yet to be stronger than the other ‘rats’ beneath the streets of London. In the cellars, Underground train lines and sewers (“Fresh and clean now after the rains”) gangs of once petty criminals fight each other for territory, and scant resources, all the while avoiding run-ins with Dr Moriarty’s boys or Fu Manchu’s Tong, and the terrible gaze of the inscrutable eyes of the aliens.
(Perhaps add Dracula’s daughters and the Hermetic Order of the Golden Dawn for some magical factions?).