I had the same thought it is very odd that they give this hint for Karlach to head to the foundry when there is nothing more than that comment. There are so many options for how this could play out. The Gondians could have a solution, the Ironhands could have a solution, maybe you need to reconcile them and they solve it together? There could be an option to take Dammon to the hells and have him do the repairs in the House of Hope of something like that, or have to retrieve supplies from the hells with the help of Mizora etc. Hopefully they will add something in the future.
I mean if we could travel to Avernus, what's stopping us from travelling to the material plane, kidnapping them all and holding hostage at point blank Fireball range?
