Negative Energy plane

The Negative Energy plane, sometimes called the Plane of Death or the Negative Material plane, was an Inner Plane or Energy Plane of the Great Wheel cosmology and the World Tree cosmology models. After the Spellplague, the Negative Energy Plane collapsed into the Elemental Chaos, mixing with all the other Inner Planes. The goddess Shar managed to channel some of this death energy into the Plane of Shadow and create the Shadowfell. Cosmologists believe that Negative Energy and its opposite Positive Energy, combined with the four elements, make up the known universe. This plane was extremely hostile to all forms of life and energy. Only the undead could travel with impunity here, and the more powerful undead could harness this death energy to attack the living with devastating effect.

Cosmology
According to the Great Wheel cosmology model, the Negative Energy plane could be reached via the Ethereal Plane or an adjacent elemental plane, but not by an elemental vortex. (However, there is speculation that the Void card in the deck of many things brought a person here.) If traveling through the Deep Ethereal, a pure black curtain of vaporous color indicated the boundary of the Plane of Death's Border Ethereal region. Such was the influence of the Negative Energy plane that the boundary with the Border Ethereal was weak and fluctuated randomly at times. Travelers thinking they were safe staying in the Border Ethereal could suddenly be dumped on the Plane of Death some distance from the rapidly receding border. Using the spherical model, this plane was adjacent to the quasi-elemental planes of Dust, Ash, Vacuum, and Salt. Temporary gates could be created by the plane shift spell.