The original purpose of the Netherworld was the act as a "limbo" for spirits to come to terms with death and prepare for the journey to the last light, but it has long since fallen into decay. In the Netherworld, nothing ever lives or dies.
When the corporeal world became overpopulated with drifting spirits, deep rifts began to appear in the æther, forming cold spots that can be found by Rhabdomancers. When the rifts became too wide, the ethereal threshold broke, making moving between the "veils" of these dimensions possible. According to Nashira Sargas, when Earth broke its threshold in 1859, it became exposed to She'ol, which in turn became accessible to the Emim.
She'ol is the Hebrew word for hell, the Netherworld, or the Underworld.