A great sundering long ago separated Zodiark from Hydaelyn, and in the process thirteen dimensional shards ("Reflections of the Source") were created. The game takes place on the Source ("this very star"), where Zodiark is implied to be imprisoned in the white moon. Each time a Calamity happens, the barriers between the worlds breaks down and a Rejoining melds a shard back to the Source, deleting that world and slowly restores Zodiark (this has happened seven times so far). But apparently there's also the Balance between Light and Darkness as a factor: if a world gets too much shifted to either end, it becomes a void. The Thirteenth World (fathest shard from the Source) experienced a Flood of Darkness and is origin of the many Voidsent we encounter. The First World (closest shard to the Source) is on the verge of a Flood of Light "erasing all form and life."

Now on the Source, we have been told that twin dooms are coming: either the Eight and final Rejoining will occur, or the Source will (somehow) experience a Flood of Light like the First World. The ascian Elidibus is especially concerned with the balance of Light and Dark, which why he took the body of Zenos yae Galvus and intends to destroy the Warrior of Light. This is apparently also the reason we "must" become the Warrior of Darkness in Shadowbringers. What makes everything so confusing is the balance seems to work independently of Hydaelyn & Zodiark and hasn't been explained clearly.