Personally, I doubt it's Shar, even though the game suggests that both she and Selune will be heavily involved in the plot.

My reasons:

1) Shadow magic and Shadow Weave aren't related, at least DnD rules say so. And if it was Shar's magic, the hag would've noticed. Plus, Gale mentions that the magic is Netherese. It's ancient and very powerful - at least it's said that nobody has been able to repeat what Netherese arcanists used to do without breaking a sweat. In addition, Netherese were very proud of their abilities, to the point they thought themselves worthy of ascending to godhood.
2) Shar manipulates differently. She evokes dark emotions, she plays with insecurities and fears, she basically breaks a person's mind to make them do her bidding. The dream person uses a more "carrot-on-the-stick" approach.
3) The tadpole is NOT happy about intervention. In the final dream, the player's character feels its anger, while the narrator says "It wants him/her gone". Also, you have the option to resist both your dream person and the tadpole.
4) No god tolerates Illithids because once a host turns, their soul is destroyed. Many followers of different deities never got their afterlife because of ceremorphosis. Shar would use a lot of secret tactics to get what she wants, but this is too much.

My theory is that the Absolute is a powerful arcanist, the one who survived the fall of Netheril and managed to preserve himself/herself. Now, they're using their magic to continue what Karsus started...or maybe even to bring Karsus back since his soul was never accepted by any of the deities and is bound to Material Plane.

Why do I think so?

1) There is a reason why Gale mentions Karsus' Folly. It's not just his backstory, there is something more about the guy who tried to kill a goddess to become a god.
2) The dress of our dream person looks very...odd. There is a hedonistic touch to it, and maybe not just to please the eye. It's said that the Empire of Netheril used to be very decadent and enjoyed luxurious lifestyle.
3) Gods are growing anxious. Some people theorize that the skeletion in our camp was sent by Kelemvor himself. He is here to make sure that we live long enough to accomplish something. Why would he be so interested in us?
4) Hells are getting involved. Raphael was right: if the PC and the rest turn, their souls would be worthless. At the same time, he can't remove tadpoles, yet he still pressures us that he is our only option, so we should just sell our souls to him. Maybe, he wants our souls, so that someone else won't get them? Gods need souls and worshippers to grow strong. What if that "let me in" thing ends may end with us pledging our soul to the Absolute without knowing it?

To sum it up: The Absolute thinks outside the box and doesn't give a crap about established rules. It unvampired a vampire, it affects physiology...and it's currently not as strong as it wants to be, hence tadpoles. So, it needs followers, worshippers and possibly avatars, so it's making a lot of noise accross the planes. It feels more like a new player than a Shar's gambit.

I didn’t invest any points into self awareness