So Gortash is a "politician raised by a demon" who has developed technology that allows him to control Baldur's Gate through an army of automata.
And Karlach was condemned to the seven hells by "politicians", and her heart has been replaced by a mechanical implement that pumps an explosive liquid through her veins.
I'll eat my Bard's Hat on Camp Party Night if these two are not related.
Hypothesis -> Gortash, somehow, was the politician who sold Karlach to the Seven Hells, taking her Barbarian Heart to fuel his automata. She is on a quest for revenge (and to get back her heart, which might turn her into a human).
Party conflict -> Gortash seems amenable to new perspectives, and convincing him to turn his robot army *against* the Absolute would be a huge asset. However, there is no way that Karlach would stay in the group with Gortash as an ally.
Similarities -> Everyone has a special nemesis waiting at the city: Astarion vs. Cazador (datamined content shows a similar conflict, where Cazador can be "recruited"), Wyll vs. his father. Gale vs. Lorroakan, and Shadowheart vs. the Shadowhearties waiting for her, Lae'zel vs. her Queen...
What do you think?
What if your playing AS Karlach and you decide to make that deal?
I do agree what was done to Karlach might be the result of what Gortash did to her, but I don't think he uses her heart to power his Automata army, I think they are empowered and run by Soul Coins. I think Zariel has Karlach heart and she will have to confront Zariel at some point, maybe with Lulu's help.