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.
I think it has been stated that the player will visit Avernus again at some later point in the story.
The Sharrans do not seem to be in a pact with the Absolute. New info on Ketheric from some dialogue responses of the Nightsong:
Thousands of Sharrans came here seeking my kiss. I am her instrument, transforming the faithful into shadows. I want to sing my own song, not Shars, not Ketherics Shar is the nightsinger and I her nightsong I am a slave of Shar Slay Ketheric, his wretched existence binds me to this temple I was captured by Ketheric, he turned me into this creature
I sometimes use thought experiments. I don't necessarily believe in every idea I post for discussion on this forum