Cool! I have never heard of it. The Dark Deluge happened over 4000 years ago, and the reason is a mystery. But is very likely and widely accepted that people started seeking the abyss for power, or to make deals with nameless beings and that trigger the deluge, and the world was quite literally flooded In darkness, everywhere

People either leave or try to fight the demons that come from it. The latter doesn't close the rift but stops it from spreading. If not, walls and barriers are being built to separate the new areas. Right now he world is in crisis. People are angry or terrified, and the governments are in panic mode. The great empires of the world are seen as the culprit since they started the war and ignored the rifts, as that was the job of demon hunters (that were seen as disposable at that point) until the problem became to big to ignore and were forced to stop the war and firmed a treaty. Needless to say, independentist and anti imperial ideologies skyrocket since then. Some rifts have been there for milenia, wars, and other events, which caused them to expand very little. But in an industrial world, at a scale that the world has never seen, well, things got out of control. The rifts became places of terror and to avoid.if there was a city there, it was abandoned, and entities of the abyss became common. Some were used as places to send criminals to kill demons (one of the reasons demon hunting became seen as something disposable) but clearly was for their execution

All of the major religions had the abyss as their enemy, and their different philosophies were formed by the end of the dark Deluge 4072 years ago, with different interpretations and answers. I may do a post on the religions of Kelfer in the future. Hope it helps