This is one of my current theories. It's quite a lot different in nature to the other ones in this thread, but here it goes:
I believe that "the Absolute" is not a single entity in the way others are thinking, i.e., it is not Vlaakith, Tiamat, Lolth, etc. It is thus possible that "the Absolute" is merely a construct / proxy.
There are a few reasons why I believe this is so:
First of all, we can see that characters have different views of what the Absolute actually is. Some believe "the Absolute" to be female, some characters believe "the Absolute" is male, whilst the mind flayer that captured us relates that: "You see other mind flayers arranged in a serene circle. Absolute unity. Absolute power", upon casting speak with dead on it. Perhaps it is because the thought of the Absolute being a female to a drow priestess from Menzoberranzan is more convincing or pleasing, as is a war-like male deity to certain goblins.
Secondly, I think it is likely that the Dead Three are behind what the Absolute "is", or somehow they are manifesting what the Absolute is in the minds of others. They have potentially done this via manipulating mind flayer tadpoles with Netherese magic in order to keep them in stasis. Thus, this enables the mind flayer tadpoles to continue projecting some sort of internal voice to the True Souls, convincing them to carry out its will telepathically. I think True Souls are those who have mind flayer tadpoles inside of them (but are unaware). Thus, the player characters aren't technically True Souls, and thus the other True Souls (Gut, Dror Ragzlin, Minthara, etc) mistake them for being True Souls. Again, they think they are communicating with "the Absolute", a brand new deity, but that is far from the truth, as "the Absolute" is merely a proxy, a voice in their heads from another source. By proxy, "the Absolute" knows that we know that we carry the tadpole, unlike the "other" True Souls. This is why High Priestess Gut and Minthara may have attempted to kill us: "The Absolute" must have demanded it.
---End of basic theory for "What is the Absolute"---
Further speculation:
The Absolute must, of course, have something to do with the mind flayers. Perhaps the Dead Three corrupted or gained control of an elder brain? This, in turn, controls the mind flayers to capture others (drow, goblins, etc), and inject them with "modified" tadpoles. With the modified tadpoles in their heads (preventing ceremorphosis), the voice ("the Absolute" proxy) commands them to carry out the will of the Dead Three. Considering the domains and portfolios of the Dead Three, they would certainly revel in the slaughter of others, such as the druids of the Emerald Grove. "Curved drow blades, crude goblin torches, gnoll teeth dripping blood" - what better force to wreak havoc and carnage with? Mayhap the Dead Three are trying to accrue followers to completely overtake others' (Shar's) portfolio / domain. As the Dead Three are somewhat powerless (potentially lesser deities/demigods) compared to their past glories, perhaps this is their bid to fully regain their divinity, with much slaughter in the process?
