After a little research today, I'm satisfied that the
Spoiler: big bad
Show
is not meant to be Fin Fang Foom at all, instead is an obscure cosmic entity that was actually called He Who Dwells Below or more directly the Dweller-in-Darkness through most of his scant appearances. If it's the same thing, he started out as a throwaway Cthulhu knockoff in Thor in the '70's and has only been plucked from obscurity a handful of times since, notably as a demonic Dr. Strange villain and more recently in the aftermath of Fear Itself.

His major thing isn't soul stealing, exactly, but causing and feeding on humanity's fear. He has used telepathy to communicate and deliver terror, apparently. He led a group called the Fear Lords, which fell apart like villain teams are wont. Seems there's some in- and out-of-book discrepancies about whether he is a spawn of the O.G. Cthulhu and/or Nightmare's cuz.

I didn't come across any prior connections to Shang-Chi or any ring artifacts, just a crown. Maybe I didn't dig deep enough, but he honestly seemed kinda boring to be plugged into the movie's story the way they did, and right now I think they just needed a really obscure not-Foom to put on screen to be killed.

I could be wrong, but those are my $.02 for now.