Mythical Origin of "Fugue Plain" in the AD&D Metaverse?

I’ve been playing AD&D for nearly three decades now, and read many of the novels, most set in the Forgotten Realms milieu, and more than once ran across the concept of the Fugue Plain: a sort of penultimate purgatory or “soul depot” where the souls of the recently slain await the next scheduled arrival of their particular patron diety who will whisk them away to whichever heaven or hell their God or Goddess inhabits.

If you’ve ever been exposed to TSR’s fantasy products, you will know that very, very little of the settings are original. Most of them is borrowed from other cultures and glommed onto the edges of whatever milieu is hot this decade… once it was Greyhawk’s Oerth, and lately it has been The Forgotten Realms’ Faerun.

Same for the “Cosmology” which all of the milieus conveniently share. The outer planes – e.g., the heavenly, hellish, and purgatorical abodes of the Gods and Demigods – are representative of many Western cultures’ mythology. Only small parts of it appear to be original myth.

So, where does the idea of “fugue plain” originate? Was this indeed a creative invention of Ed Greenwood, et. al.? Or is this another element borrowed from some traditional mythology… and if so, which one?

Perhaps Dante’s Inferno? If I recall correctly, outside the Gate of Hell, there was a vast plain. While waiting to enter Hell, Dante encounters the lost souls of those who chose neither good nor evil, but only their own self-interests. Upon entering Hell, the first thing he encounters is Minos, the Judge of the Dead, who decides each sinner’s punishment.

Of course, Dante stole most of his stuff from ancient Greek and Roman mythology. In Classical myth, a soul would enter the underworld in an entry depot, where Minos would judge him and send him to Elysium, or Tarterus, depending on his virtues and vices.

Fugue plain? I like my fugue with strawberry jam.