Exodus means a mass departure of people. In this case however, Exodus was not just a term. Exodus was a term used to call the event that unfolded before the Obelisk of Goap's Outbreak.
Tens of thousands of Demons that resided in BA-22 mysteriously disappeared one day. Because of this, the Kingdom's Royal Mages were sent to BA-22 to investigate; they were afraid an Outbreak would occur.
However, upon investigation, it was revealed that the Demons had massively moved to somewhere else, and no one knew where they headed. This bizarre and confusing event was called Exodus.
It was only after the Obelisk of Goap's Outbreak did everyone know where the Demons had moved to: Death Howl Dungeon.
They had hidden themselves there for a few weeks, waiting for the opportunity to show themselves right when the Outbreak occured. Needless to say, they were successful in their endeavor.