So I feel this is a plot hole and makes no sense in Act 3. That there is no way to truly save Karlach. In the scripted stories ending saying something like we tried everything, which is bull dung.
The options are Romance Karlach an go to Avernus with her. Or send Wyll with her if you don't romance her. God Gale can make Karlach ascend and she can become a goddess but I believe you have to hook Karlach and Gale up together. And that's only possible if you don't play a custom Tav. Have her blow up, or she becomes an Illithid. None of these are the endings I seek, they are all bad endings for Karlach.

So here's the plot hole that Larian should fix by adding this to the game. Steel Watcher's are made from Infernal Iron. They drop it so they are obliviously made from it. They are made from the same metal as Karlach's heart ( Infernal Engine). An for these things to run they most likely have Infernal Engines of there own since there using infernal iron. (Yes I know there are brains an a wireless system running them to) Yet they have no problem being in the material plane. You can even get Enriched Infernal Iron from the Hellfire Watcher's. (I thought Dammon could save her with Enriched Infernal Iron, but nope.) Anyways. If you save the Gondian Gnomes in the quest Save the Gondians. You should be able to introduce Zanner Toobin to Dammon. Since they can both work with Infernal Iron. And Zanner Toobin could teach Dammon how he solved the problem for the Hellfire Watcher's Infernal Engines and Dammon could then make the part for Karlach. Allowing her to stay in Baldur's Gate Since she does help in saving the city. Larian please add something like this. It's not right to abandon Karlach. You even lead the player on by allowing us to say there might be another way to save her. Which there is no option to save her if you play a custom Tav.