Ethereal Planes are celestial dimensions that parallel the physical realm of each universe fashioned by Eru The One and his associates. From here the Implementors implement, and share their home with many other supernatural and fantastical beings.

Each universe is made up of multiple Ethereal Planes, all connecting to a section of the Outer Halls designated for that respective universe, which in turn is connected to the Inner Halls. There are at least two known Ethereal Planes, that of Atrii (which parallels the planet of Zork), and that of Irina (which almost nothing is known about).

Using either divine or magical means, physical beings have managed to ascend into these Planes. The only being, however, who is known to have entered beyond and into the Timeless Halls is Zylon the Aged.

SOURCE(S): Beyond Zork, Wishbringer, The Zork Chronicles, A History of Quendor, Zylon the Aged, Zork:Nemesis, Zork:Grand Inquisitor