The Not-Mother was a malicious paranatural entity that inhabited a dimension called "Temple" that was accessible through the Slide Projector. After the Slide Projector itself, it was a secondary cause of the Ordinary AWE.
When Tom Barlow and his gang stole the Slide Projector from Jesse Faden, who with her friends had originally discovered Temple and the Not-Mother, they entered Temple themselves, unaware of the danger. The Not-Mother lured Tom's group into drinking her "milk", turning them violent. The morning after Tom killed his math teacher and was arrested by the police, all of the adults living in Ordinary disappeared. It is not known if this was caused by the Not-Mother herself or Jesse, who had wished the previous night for adults to be gone.
The Not-Mother continued seducing Tom's group and transformed them into monsters that Jesse called "Dung Monkeys". When Jesse and several other children of Ordinary stole the Slide Projector back, she figured out how to turn it off with the help of Polaris, which made the Dung Monkeys and the Not-Mother vanish. Jesse then burned most of the slides, including the one leading to Temple, likely ensuring the Not-Mother could never be reached again.