The Well of Forever is a legendary location where all planes of existence overlap and meet at the same location. The exact location of the well in any meaningful expression of euclidean space is entirely unknown (and perhaps meaningless). Even so, it's influence spreads throughout the different planes of existence. The actual well itself is a vortex within a pocket of reality at the focal point where the layers of reality come together matter and energy breeching into the well can travel to any place in any reality at any point in time.
Physical Properties
The well itself is an intense distortion in space-time, far beyond the black holes at the centers of the largest galaxies. It is a black hole so intense that it not only collapses space-time into its core but also draws in other planes of existence and even realities of the multiverse into a singular point. The tidal forces alone disrupt and destroy any meaningful structure to matter that crosses its event horizon while energy is similarly disrupted and torn apart losing any sibilance of cohesion to its original form.
Openings
The unique properties of the well prevent the crossing into other realities directly, however, through unknown means there have been found openings to the well in various planes that connect to other planes of existence. These entrances are rips in the fabric of space-time that allow passage between planes through complex and ever-changing conduits and tunnels that form as reality breaks down between the different planes and areas of the multiverse. The openings are unstable and shift randomly throughout space-time.
Navigation through the well itself is impossible, anything that attempts to directly enter the well is torn apart and its energy spread across multiple realities and planes of existence. There is only one way to effectively use the well to travel through the well intact, that is through the Verum Transitus, which sits within Void Space on a point constructed specifically to anchor it to the well. The structure is constructed with powerful magic forces that bend and shape the structure to interface with the well. Those passing through the Transitus into the well can navigate the complex tunnels of reality that allow crossing through between planes, realities, times, and locations in space.