When Linda first arrived in White Dove City, he spent much of his time at the library, gaining a general understanding of witches. In the context of the game, the contracts, when applied to reality, were essentially "young girls harboring intense, desperate wishes who were lured by the Demon King’s whispers into signing away their souls."
In other words, witches were the Demon King’s subordinates—cursed existences.
Wherever they traveled, cities were razed and the land was littered with corpses. When a witch reached the end of her life, she would transform into a "Fallen Beast," triggering a catastrophic, inhumane disaster.
Chapter 22