Negative Energy Plane
The Negative Energy Plane, formerly one of the Inner Planes, was a dark void of unlife and endless night. Shortly before the Deity War, Kiaransalee somehow managed to bring a fragment of the plane into the Material Plane. When the fragment was destroyed, it left behind a tiny hole in the planar walls that leaked negative energy into the plane of shadow. Ultimately, the chaos of the Deity War ripped the hole wide open, causing the entire Negative Energy Plane to drain into the Plane of Shadow, resulting in the creation of the Shadowfell.