It has to be some sort of god.
Why? There are Clerics of the Absolute. You literally can't be a Cleric unless you well... follow a real god. Unless this has been changed somehow for BG3 specifically.
Perhaps Shar? I believe this goddess pretended to be someone else. There are other signs pointing to Shar as well.
In the Forgotten Realms, there has been a few cases of none-god being worshipped and their followers receiving spells because that person's godly patron was granting them. Those none-god would all qualify as Chosen. So the Absolute doesn't need to be a deity herself, but it does mean a god is involved at some levels.
I can't decide if Shar is in it or against it. She could have had followers corrupted by the Absolute too.