If we can suppose that Ceremorphosis not only destroys the host's soul, but in doing so provides that soul's "essence" to the newborn Mind Flayer... and maybe ultimately to that illithid's Elder Brain, then I have a twist on these Absolute theories.

My guess is that The Absolute is: An Elder Brain that has absorbed or been altered by Amelyssan. This makes a lot of things fit. Perhaps she survived the destruction of the Throne of Blood and got picked up floating in the Astral Plane by that nautiloid or the Githyanki?

New Elder Brain Melissan is preparing to, or maybe already has, eaten the brains and souls of Bane and Myrkul (who were quasi-divine mortals on Toril, I believe.) It could be something she learns can be done to the Chosen or weakened mortal forms of any deities.

Anyway, in keeping with the saga, it would be traditional if Bhaal mojo is the missing ingredient in her stew when we begin the story.

Amelyssan could be the prophesied Illithid "Adversary." They ate her, but she retained her personality or will to some degree. She discovered that Ceremorphosizng the Chosen or the Quasi-Divine is a way to get what she wanted previously: To keep the Bhaal essence for herself and become a Goddess.

This time around, she's taking power from all of the Dead Three. In a way, the Absolute is becoming all of them as they are "absorbed." It can be simultaneously true that the Absolute is Bane, Myrkul, Bhaal, anyone else she's eaten, "she," "it," "he," "they," etc.

Quite possibly other deities' Chosen, or weakened Quasi-Divines are potential targets, which could tie in some Origin Characters. (Selune? Mystra? Bane's subjects like Maglubliyet might also be on the menu for the new Super-Illithid-Amelyssan too..)
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I am not sure if the resistance to Ceremorphosis is a part of her plan, if it is a natural resistance that Chosen types have, if it is a protective measure by another power, or if it is part of a Githyanki plot to sterilize all Mind Flayers. (???) At any rate, She really wants you to break through that resistance and let her tadpole take over.

Some clues that I think support this:
- There are authoritative sources giving gender as "She" and also "It/Sexless." Both of these can be true, in a way, if Amelyssan's mind/will has taken over an Elder Brain. Also fits the narrator is the villain tradition.
- The Absolute Talisman image seems clearly a merging of the Bane and Myrkul symbols.
- The keen interest from Diabolical higher-ups/Raphael. I suspect it messes up contracts for souls if these souls get "eaten" by mind flayers.
-- Perhaps Amelyssan or an NPC would actually be evading a contract with a Devil this way. This could involve Gale and/or Thay. I believe Ssass Tam had a 1000-year term with an Archdevil on his soul.

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I am not an FR Loremaster, and this theory needs details, but I think it's close.