After confronting Gortash with Karlach in Wyrm's Crossing he returns to his mansion in the Upper/Noble district forcing the group to hunt him down.
Being the inventor he is the whole mansion should be a steampunk deathtrap filled with traps and watchers. As the party manages to fight their way through the maze they find Gortash in his main room surrounded by improved/monstrous watchers that you need to kill first to take down the magical shield protecting Gortash.
As the battle comes to a close and Gortash lies in the pool of blood Karlach gets ready to deliver the killing blow, Mizora shows up and takes him away to Avernus forcing the party to follow them through the infernal portal that she leaves behind. There we are confronted by Zariel and after a short battle that we loose we are given options. If all persuasion checks fail you are forced to abandon Karlach but if not you end up making a deal.
As the deal is struck Karlach is freed but at what cost? Tav is forced to take her place in Avernus after the illithid threat is gone, serving Zariel for two years. Once those two years have passed Tav comes back to find Karlach married to someone else.
And now I'm going to slowly leave this thread before Orchid hunts me down and tries to kill me LOL
I'm so sorry I couldn't help myself. I had to pop back in and do it, I had to lmao.
It would be interesting to have an option to make a deal with Zariel for Karlachs freedom. Maybe depending on the players choice it could lead to Karlach being 'cured' of her engine but is forced to sell her soul which would mean that she can stay with Tav but when she dies she'd be going back to Zariel. (this also opens a possibility for Karlach and Tav to find a way out of that deal in the future)
Or have a quest (something like the House of Hope) where Zariel task you to do something for her in exchange for Karlachs cure/freedom.