The Day After Ragnarok: One Snake Cult, One Ranger
In 1945, Nazi sorcerers tried to start the Götterdämmerung – the end of the world as told in Norse mythology. Nazi explorers and arcanists had recovered new texts from a cave complex in Norway that finally allowed their sorcerers to complete a ritual that would summon Jörmungandr, the Midgard Serpent. The Nazis planned to start Götterdämmerung but with themselves in key roles – they would win WW2 by default and be the rulers of the new world to come. Some of them would even be the new Aesir - a new generation of gods … or so they thought.
It worked up to a point – they DID manage to summon the Midgard Serpent. However, President Truman took an action they did not forsee. He ordered a nuke strike on it. A few days after the Serpent appeared in the ocean, but before it could encircle the entire world, a lone US bomber carrying a prototype A-bomb flew into the serpent's eye and detonated its lethal cargo, killing the serpent but causing further destruction.
The serpent's carcass is now draped across north Africa and much of Europe (about where the Iron Curtain was). The fallout from the blast mixed with magical serpent blood and venom and fell in various places in the world and reduced much of the US east of the Rockies to wasteland that is only now beginning to recover. The fallout mutated animal life or spontaneously generated some, so we now have monsters, undead, and creatures of myth to deal with. Magic now works, and inventors are creating mad science with items harvested from the enormous carcass of the serpent.