afroakuma's Planar Questions Thread! (You ask, I'll answer)

[Zubrowka74]

Well, shadows (the undead) aren't Shadow (the substance); their nature merely attunes them to that plane. Most natives of the Plane of Shadow are living beings, of a sort, though the dark channels where the shadows (again, the undead) run thick, the doldrums, give rise to potent forms of "pure" undead that rise as a counterbalance to life.

I think at least one edition had the shadow undead come from the plane of Shadow. 2e ?