There is another long thread about this but I'm posting here until they get merged

As someone who also once believed the Absolute was a 'her' -- others have shown me that other NPCs refer to the Absolute as 'him' so, like "Daisy" your dream lover, the god looks however you want them to look.
I originally missed the dead three iconography because I somehow missed that Bane's hand is now open. (which is strange -- the image of a Iron Fist seems to better invoke tyranny) Anyway, the dead three are good possibility buuuut that doesn't explain either the Shadow Magic or the deception.
I'm thinking that the dead three and Shar working together is the best explanation out there. Shar explains the deception and shadow weave and the dead three best explains the iconography. Shar has a history of working through other gods -- such as the decision to use Cyric to kill Mystra. Indeed all the (original) evil gods are her creation.
Ioulaum explains mind flayers and Netherese magic buuut Ioulaum was linked to Mystryl's weave not Shar's shadow weave. (I kinda wish the game wouldn't conflate shadow magic with Netherese magic. There was only one Netherese city that used Netherese Shadow magic)
Datamining -- super spoilers
It also explains the 'weapon' that keeps getting referred to. We know the current Nightsong has a dialogue where she asks the adventurers to free her. The old Nightsong is no longer as useful and I'm guessing that Shadowheart was supposed absorb the Gith soul in the d20 box and become Nightsong 2.0.
I even wonder if Vlaakith or Gith herself could be part of the Absolute consortium. If the soul in Shadowheart's box is Zerthimon and he can be controlled using the absolute's psychic powers then "the weapon" could unite the Gith factions and Vlaakith would have Absolute control over the Gith peoples.