Yes well, as i said, and that mural throws in our faces as the first thing we see - the supposed Illithid deities should be considered. But as that lore says neither the more official deity, nor the other one are very clear even to Illithids themselves and remain cagey, mysterious and somewhat hidden. Connections to the Astral plane abound.
What we have seen so far in the EA points out to a more complicated plot with several forces involved. Which i prefer and like so i hope the main plot will continue on that trajectory.
However, if one would seek a malevolent force much greater and older then the Dead Three, and with even bigger grudges against everything, that would be Shar. Who also already pulled off "pretending to be other god" trick to exact her vengeance. And also - "Shar eventually made allies of Bane and a patsy of Cyric in 1374 DR. Together, their followers attempted to conquer Myth Drannor "
Her bitter fight against Selune and the whole universe would fit with the name "Absolute", as in someone who denies other forces are as great or as important.
Also;
"Shar killed Ibrandul, a lesser deity of caverns, dungeons, and the Underdark, as an act of pure opportunism. She continues to grant spells to clerics in the name of Ibrandul, reveling in the deceit of the entire affair."
What do we get to play with immediately? The Underdark - right under the defiled temple of - Selune.
What is changing the tadpoles? Shadow magic.
Auntie Ethel being too scared of it to even touch it... and who would frighten such a hag, eh?
Plus, fool Gale thinks he was so special Mystra herself came down to sex him up. Ha. Ha. And what did he get out of it? An orb of magic eating - cities or continents destroying Netherese - shadow magic - primordial weave - from an Astral plane.