They've had an idea for the story of the world for a long time. We've gotten pieces of information here and there and have built up a lot of theories. The Dev's are purposefully withholding telling us too much about the world and it's story because that's a core aspect of the game. You are meant to discover things and be driven to explore by curiosity.
If you look at the
about section of the website you can get a good rough idea, and there was some
writing updates done on Kickstarter that showed some aspects of life in SG.
An excerpt detailing the global world description of Sui Generis:
"On a treacherous world with a tortured history the meagre remnants of humanity live in awe of a misconceived past, haunted by forgotten gods and fearful of the very ground they tread. A vast sprawling underworld ever present below their feet, baleful demons lying in wait. Thaumaturges, people with powerful psychic abilities, have abandoned their once benevolent nature turning to cruel and dark activities in their quest for ultimate power. Awoken by impending threats, an abandoned and damaged being gives rise to its ancient weapons who now walk amongst the people again."