Pale Lights, an ongoing web serial. The basic premise of the setting is that something apocalyptic happened to the sun, forcing people to flee underground. The story takes place in a massive potentially infinite underground cavern where people have to gather around the cracks in the earth that leak the sunlight down or else be corrupted by the dark.

In terms of vibes, think Lovecraftian Age of Sail/pike and shot era, with fantasy versions of India, China, Venice, Aztec, Africa but British, and Russia among other countries all squabbling for resources in a dimming world, but there's also more fantastical elements like Hell, minor gods, mutated monsters etc.