Karlach is one of the most beloved characters in Baldur's Gate 3, with her story resonating with a whole bunch of people. Part of her quest is finding Infernal Iron to repair her heart and then finding a blacksmith who can actually install that Infernal Iron, which means they'd have to be another Tiefling. Luckily there's a Tiefling you can find throughout the game who can help you fix Karlach.
How To Find Dammon
You can find Dammon in three different places throughout the game. He's in the Emerald Grove at the start of the game and will stay there until you complete the series of quests involving the Goblin Camp. Once you do those quests he'll move on with the rest of the Tieflings, and even if you do give him the Infernal Iron in the Emerald Grove he'll tell you to find him later, in Baldur's Gate itself.
Luckily you don't actually have to wait that long to find him, however. You can find Dammon in the Shadow-Cursed Lands during Act 2, working in the Last Light Inn. If you want to complete his quest (and thus Karlach's quest), make sure you keep Isobel alive. After this point in the game, you'll find Dammon one final time in Act 3 of Baldur's Gate 3, working as an apprentice in The Forge of the Nine.