The Ethereal Plane

Sitting between, above, below, and inside the Rings of Law, Colors, Chaos, and Elements, the Ethereal Plane often appears to visitors as a stretched and shimmering grey overlay of whatever plane is closest. Many spells allow creatures to temporarily travel here, and any teleportation magic uses this plane as a medium to move objects and creatures along.
The Ethereal Plane is transitory in nature, and is not accessible from The Astral Sea, The Unmaking Never, The Animatic Plane, or The Mortalic Plane, as they each sit outside the The Ring of Elements, which is as far as the Border Ethereal stretches.
The Ethereal is often the glue which holds incorporeal undead and other creatures together, as they tend to inhabit both this plane and another simultaneously. The plane itself is well-studied, and the plane itself seems to fluctuate in temperature depending on the season. Deadly weather events in the Ethereal Plane can vary from emberstorms in the summer months to needle-sharp icestorms in the winter. Within the plane closest, these phenomena often manifest as spontaneous portals to other planes within the same ring, such as the Shadowfall storms that sweep ships from Enos to The Grey.
The plane is thought to be the conduit on which souls ride to their final destination after death. Whatever the case, scholars agree that spending any real amount of time here is wholly reckless, and thousands of studies have proven that creatures trapped in this plane slowly lose their senses of self, shape, and existence.