Forum member Priest had some good theories. If you can find that thread it's worth the thread necromancy.
Also check out youtuber HarbsNabs (when he isn't talking about this forum). Right now I been convinced it's the dead three and the mindflayers are being used by them. The thing I haven't figured out is what role Shar has to play in all in of this. Is she working with or against the dead three?
We know that shadow magic -- Shar's magic -- is preventing the transformation from taking place. Does Shar want to get the players get the Illithid powers and serve her? Is she essentially stealing from the dead three? Or is she part of the consortium that makes up the absolute? Is she an ally to the dead three? Opponent? Can't figure that one out.