Okay, I still haven't found it. I spent the better part of an hour paging through the 300 page Isle of Dread book looking for anywhere it could be hiding. Other than bringing back memories of when my group played through this adventure, I came up empty handed.
Here is the info I have on Charontlis:
Charontlis - Demiplane
Charontlis was created by a vanished race of powerful psionics, who created it as a contemplative retreat from the world.
A cavern approximately 3 miles across, with a grassy meadow at its center. In the meadow is the fortress/palace where the creator's dwelt. Surrounding the meadow are fungal forests where the descendants of their myconid servants dwell.
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A one-way entrance to this demiplane is the gate at the top of the Iron Obelisk on the Isle of Dread, and deposits travelers in an open area in the grassy meadow outside of the palace. (The inhabitants used their own powers to exit this realm).
The myconids of the fungal forest in the Vast Hollow under the Isle of Dread are also descendants of the ancient myconids who lived here and served the creators.
Since it can't find any connection between Charontlis and the Iron Obelisk, the third paragraph (and possibly the fourth), were clearly me making up a connection. I had made the obelisk host 9 portals, and it appears that I randomly rolled the plane they each led to (that's definitely what I would have done).
If I rolled up "demiplane" for one of the portals, it's possibly that I created one. I tried looking at the random demiplane tables I have--because I would have certainly used one of those. No help from the Manual of the Planes (mostly planar traits). The 2e one could have generated the size and the connection to myconids, and it's possible I could have rolled illithids and just changed it to "vanished race of power psionics", but there's no purpose entry for me to have gotten "retreat from the world from" or anything that could have given me the terrain.
I tried looking at other sources I have and might have drawn on, like the 5e DMG random dungeon generator (nothing there), or the Ghosts of Saltmarsh random mysterious island generator (which could have given me the size, but nothing else, and I don't think I had that book yet).
So that's where I'm at. I really don't want to claim I created it (even in my own mind) without being sure, but I'm drawing a blank as to where else it may have come from. I guess I'll just label it with "Source ?" and call it good.
