I mean you just have to fix her heart engine thing with two pieces of Infernal Iron with Dammon and she can progress to the next stage of the relationship.
To clarify, it's two separate Upgrades that are needed, in two different cutscenes. Then you can slap booties.
I wish...sadly, I stumbled on to some spoilers. Folks were great about using the hidden text, but context still gave me enough info to realize that the end game for Karlach isn't what's expected. The dialogs and events make it seem like technical upgrades are needed for her heart and all will be well. But those were dead ends by the end of Act 3. You will not experience what you think you'll experience. That's all I'll say here if you haven't already been spoiled.
