River Styx

The River Styx (or just Styx), also known as the River of Blood or the Silver Path, was a transitive, cross-planar corridor that connected the Inner and Outer Planes. It was guarded and traversed by a god of the dead known in some cultures as Kelemvor, and in others as the Raven Queen of Death.

Safe travel upon the River Styx was impossible for any being without the guidance of the god of the dead; those that attempted to do so usually drowned and became ghosts trapped in the Ethereal Plane.