Previous to the Revelation, the world was ruled by dark and oppressive beings who claimed to be true gods; in reality, they were extradimensionals of great power whose evil and selfish natures tainted the entire planet. Some of them were what would today be called demons, but many had other origins.
The false gods were destroyed and/or banished by the Sept during the Revelation; but it's difficult to destroy a god, even a false one. The false gods had ruled for so long that splinters of their essences were entombed in the very fabric of reality (what arcanists and theologians call the Tapestry). There, they slumber, unaware, undreaming. In recent years, some arcane researchers have found ways to call out to these vestiges of power, and wield a portion of them in normal reality. These vestige-binders are considered heretics by the Church of the Sept, and they are tangentially mentioned in the Condemnation of the Infernal produced by the Ecumenicon of 4919. The infernally-allied noble houses who were the targets of the Southern Crusade were said to have god-binders of this sort amongst their numbers, but public information is scant.