As lore about the matter says, there are a sector of Mithadir (third layer of Arborea, one of the planes where gods live), a sector of the upper planes full of desert with athasian elves on it
I put there a full avangion who only drive in life is to bypass the gray barrier to take souls to their final rest, especially athasian elves, altough the process gets too difficult so from time to time some dangerous athasian creatures are gated there as well
As his efforts, there are a spelljammer ship near (of a previous trying to pierce the gray barrier mission), and the first of the trees of life -a really big one about 100 metres tall- guarded by a host of angel like creatures specially human sized or taller hound archons (they literally look like “swole doges”)
There is also a portal there near to the gray barrier outside Athas sphere, who must be traversed by special means when local conditions weaken it a little the really strong barrier (even mayor gods have problems traversing the barrier)
Is this canon or head canon? If canon, which Planescape book is it described in?
Seems to be mostly head cannon.
Per Planes of Chaos, the 3rd layer of Arborea (Pelion) is called Mithardir by the elves, and it contains rumors that runner elves from someplace called Athas have been sighted on the layer.
But the details (Tree of Life, portal to the grey, etc) don’t appear there.
It´s canon that there are athasian elves somewhere on Mithadir (Pelion) the third layer of Arborea, and also that Athas is almost unreachable via spelljamming ships
Is also canon that Arborea itself is filled with all sort of dangerous monsters (mostly in wilderness areas not in cities), so athasian elves in Pelion also have them, and they are probably from Athas, too
The first tree of life and the portal to the grey, and the avangion who bring the athasian elves to Pelion via a spelljammer ship are things i added to my campaign, use or discard them as you see fit
I have to believe there are out of the way ways for other planar being to get to Athas, it’s just that normal routes are not open… It’s a bit like Ravenloft in that regard. If there are ways in, there are ways out, but not by the normal planar means… Also there are green age mysteries around that still hold relics that could give access to the planes. The planes in my campaign are a bit like routing your call many times through different places to avoid people detecting where the call came from.
We are dealing with out of print fantasy, lets make our own canon, until Wizards get off their duffs.
I usually like to think that there was a time in the great past where the crystal sphere and planar connections were much more standard (green age). Then great work was done for REASONS to close the crystal sphere and planar connections and that is what caused the destruction of the seas. However this left a number of permanent gates still functional.
Even then high level magic and psionics has ways of cheating the system. The planescape mentions of Athasians are the results of those types of situations.
For my campaign’s canon Athas was accessible via the planes in the early Blue Age.
A planar accident caused energy to spill into both Athas’ border Astral and Ethereal from the Negative Material Plane. The result was the creation of both the Gray and Black.
I go back and forth with the accident being caused by a living vortices reaching into Negative Material Plane. I further toy with if living vortices are natural creatures or life-shaped engines created during the height of rhulisti civilization.
I recently (as in last weekend) adapted the Pyramid of Jenkel (from Dungeon #23) for my campaign, keeping the portal to Tarterus/Carceri. I had Demodand/Gehreleths be incredibly reluctant to come through the portal because as bad a prison as Carceri is, you know that you might eventually get out. Athas by comparison is like a Super-Max prison.
It’s arguably a toss up as to which is more difficult to escape: Athas or Ravenloft. High level psions, clerics and wizards probably have an easier time getting out of Athas but its still bloody difficult. Ravenloft is more whimsical as to what it lets out, when and why.