I really cant remember the pre-requisites for it but it is there in the game at certain permutation when you can
ask and let Karlach decide on her own if she wants to go to Avernus or not and is based on her approval rate what she will decide.
Just really a shame that there was not enough time to add hell zone, don't know if it was going to be Avernus or just some other portion of hell but it was going to be about Vlaakith's temple area of sort, is really a pity it ended up getting cut out would have had a chance to get Karlach's story more fleshed out for sure.
I never bookmakered it but it was when people were talking with Sven Vincke and there was a lot of talk about stuff, places, visions they wanted to add into the game but didn't make for various reasons such as lack of tech to support it properly or just slowing down the flow of gameplay too much or simply just lack of enough time for it, another example was having more places to see inbetween the transistion from Act 2 to Act 3.
So it really shows how lacking Karlach, Wyll (too many rewrites) and Minthara suffers from, what I find most odd is why is Karlach still not added onto the official banner/poster as part of the beloved origin team?
