After the Vampire Apocalypse
Two generations ago, they came. Nobody knows where they came from, but they could spread, create others. Vampires. Vampire spawn. All over the world. Humanity was doomed.
Yet a few people escaped into isolated environments, which they could protect. The vampires have one key weakness, namely sunlight. Not just sunlight, but ultra-violet light. It's possible to protect communities with high-powered UV lamps. Possible at least as long as there's electricity to power the lights, and the technology can be maintained in the absence of manufacturing or the infrastructure of the modern world.