I suspect it's [something representing] The Dead Three. My theory is as follows:
1. Load screens indicate that the Dead Three are something important, yet we've seen nothing about it. 2. The game ties in [somewhat] to Baldur's Gate 1 & 2, Bhaal is part of the Dead Three 3. The name 'Absolute' suggests something that 'covers everything', something combined 4. The forces we encounter have no problem with the portfolio's of the Dead Three 5. We obtain the services of something talkative that serves Jergal and/or Kelemvor. Jergal is also directly responsible for The Dead Three. 6. There are clerics of the Absolute, indicating something [at least remotely] Divine.
Then. I highly, highly suspect all origin stories will tie in to the main plot and all will find an enemy in the Absolute, and perhaps some will find an ally. Yet I doubt ANY of these are the actual Absolute.
And we've already encountered most of the opposing forces. Avernus [Zariel] - Raphael is introduced as the if-you-fail-at-other-things option. There are quite a number of mentions/depictions of the Bloodwar and [Descent into] Avernus. It's bound to be important. Demons [Wyll] - He comes with a Demon overlord. One that might have a vested interest in the above mentioned Bloodwar. Githyanki [Laezal] - Tiamat once held Zariel's position, again, in Avernus, currently allied with the Githyanki. Sharrans[Shadowheart] Shadowheart directly opposes the Githyanki, they weren't chasing Mindflayers, they were chasing Shadowheart and the... Relic of Holies. Vampires [Astarion] - His origin will undoubtely be the choice of 'side with Vamps and overtrhow the other evil, or be more evil and overthrow the Vamps' Selunites [Default Tav] - Also mentioned to be in a war against Sharrans, their stuff is broken everywhere.
This leaves Gale. Personally, I do not believe Gale's story in the slightest, and I suspect him to just be [unknowingly] teleporting magic stuff to someone else. He'd need 2000 years and a pointy hat for me to believe his origin. Or perhaps that is his destiny or something? He's Chosen to go all Catastrophe on the Absolute?
The 3 in the game title is based on the 3 floating towers in Avernus that launch imps in the intro cinematic.
Technically, Wyll's patron is just an neutral evil outsider as Succubus aren't demons in 5e anymore, just fiends. She could be working for anyone evil.
Going by Astarion
, the vampires could be allied to Avernus as his master engraved an infernal poem on his back. He also seems to worship Shar.