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Although the Ascians come from the North and Erebus is based in Antarctica, they are actually the minions of Erebus. Their name means "casting no shadow". They cast no shadow NOT because they are from the Equator (which they are not, or at least not entirely), but because they are from Erebus, whose name comes from the Greek elemental Titan who was "The Darkness of the Underworld".
My intent was to suggest the Wall of Nessus was needed to defend from
enemies on all sides. I take the story of tall pale warriors riding in a
ship with a deep voice coming from below the river as a sign of collaboration between Erebus and Abaia. As he gets the most mention I guess I tend to think of Abaia as the big boss of the 17 megatherians and was shorthanding with regard to his leadership of the Ascians.
I don't associate the term "Ascians" specifically with Erebus, despite his mythological namesake. It is actually a real archaic term given by Europeans to people living near the equator and we learn it is falsely given to the enemies of The Commonwealth who are really from the equivalent of N. America.
My impression is that the Ascians are somewhat equally enslaved by a collaboration
of the megatherians, hence their loyalty to the Group of Seventeen. (surely allegory;
even a Byzantine government is seen as superior to the mind-numbing sameness of communism).
Mount Erebus in Antarctica does suggest a home space for Erebus. Most feel Abaia lives in the ocean, but since the mythological Abaia was specifically a fresh-water magic eel, I must think his primary residence is Lake Diuturna. Juturna's name and apperances and the island people's belief in Oannes would seem to support this idea.
I think Wolfe making his monster names a combination of Melanesian (Abaia), Greek (Erebus), S. American (Jurupari), Norse (kraken) Persian (Jahi), Borgean (peryton) and Judeo-Christian fallen angel (Arioch) is highly significant.