I don't recall where I found this image, but it seemed to fit the theme.
We've had to cancel two weeks in a row due to circumstances outside our control. I have however, had time to customize the setting some. In my version of the Black City, we're basically working under the assumption that everything is true. Conspiracy theories about ancient aliens, aliens (really core to the setting anyway), Atlantis, reptilians from the center of the earth, alternate dimensions and parallel worlds, Stargate type stuff, Lovecraft, mythology, everything.
The campaign takes place in a fairly realistic Viking age setting, until you get to the Thule islands and the black city. The players are, to the best of their abilities, playing their characters as Norsemen of the time period, with appropriate weapons and armor (I made a table to roll on for equipment at character generation, it saves a lot of time). I have made some concessions to magic, for what the Norsemen believed in, and added some flavor to the rules set we're using (LotFP, as suggested in the original Black City campaign). I added a couple of flavorful house rules too that seemed appropriate for the Viking Age, notable “Shields Shall Be Splintered” and “Death and Dismemberment” from the OSR blogosphere, and Berserker and Gothi classes, and some Skald skills.
In my head canon the Alien Greys that built the city died off, but their creations didn't. I have them as the creators of the Hyperboreans, who outlived their masters, and then tried to enslave the rest of the human race way back in prehistory. My Hyperboreans are essentially a human sub-species designed to act as overseers for the rest of the human and proto-human slaves the Greys kept. Tall, pale, cold resistant, magically adept, and psionic. Hyperboreans are familiar with the use of most Grey technology, but cannot make it, being (genetically manipulated) humans though, they are creative enough to effect some repairs, and jury rig some new devices, as well as use them in unintended ways.
Next come the Atlanteans. Also human, not genetically manipulated, descended from human slaves that escaped the Hyperboreans and settled Atlantis, they are tech users, which made them far more advanced than their Neolithic and Bronze age contemporaries. They have a whole gonzo Steam-Punk Tesla meets Magic with an ancient Greek aesthetic going on. Their tech is their own, as is their magic; different than that of the Hyperboreans, who are basically just scavenging from the Greys.
I mention both the Hyperboreans and the Atlanteans because, long after the Greys died out, they fought a long genocidal war against each other in my head canon, and one important front was the Black City. So their artifacts also litter the ruins, and they are responsible for some of the destruction of the environment there, and some of the deadly hazards of it too.
The Reptilians from the Earth's center, I am less sure about yet. I might want Silurians from Doctor Who, or maybe Sleestaks from Land of the Lost. Maybe both? Possibly neither.
I may or may not consider Robert E. Howard's Hyborean Age canon to the campaign, I haven't decided yet, but I'll have to slot in some other ancient stuff if I do. I am trying to slot in an old school Battlestar Galactica reference (The 13th Tribe of Man, “Life here began out there”), but need to reconcile it with the rest. My current thought is to use the Warden from Metamorphosis Alpha as the colony ship from Kobol, possibly some time travel shenanigans shoe-horned in to make it work logically in my mind. It would give us yet another ancient spaceship wreck somewhere on earth. Anyway, I want Colonial Warriors and Cylons to make an appearance, if only as an “ancient aliens” style background.
The dimensional gates make things like side trips to other campaign worlds a la Q1 a possibility, actually looking forward to throwing Carcosa in the mix. Thinking about adding some bits from ASE. I know I want dinosaurs at some point. Maybe some other time travel shenanigans so I can bring in Nazi occultists, possibly with a diesel-punk moon base.
The setting already has competing mad AIs and Alien ghosts and so much more.