Blueprint and illustration of Karlach's infernal engine.
So much of Karlach’s engine is hand waved away by the game and a lot is left up to our imagination, and I really wanted to figure out how she could access her engine and install upgrades and how she consumes soul coins.
Since the Steel Watchers identified Karlach as an out-of-date Steel Watcher and I think that provides some key insights to her engine and how it works. Firstly, I think this indicates that the engine is an early prototype developed by Gortash as part of his deal with Zariel. And secondly, Steel Watchers are classified as constructs so I don’t think it’s too much of a stretch to see her engine as one as well, even if only a minor one. This is further supported by the upgrades Dammon makes which grant Heart of Iron and Heart Ablaze both being consistent with Constructs being unaffected by the ‘Charmed’ condition.
If her engine is a minor construct I think some of the magic it's imbued with, outside of Dammon’s upgrades, are the ability to allow her to phase through her chest to access her engine, most often for maintenance and soul coin consumption. If you look closely at the outward glow of Karlach’s engine through her chest, you don’t really consistently see a sternum or the costal cartilage that would usually be over that area of her lungs and heart. Her tiefling ridges kinda obscure this. I headcanon that as part of the procedure to install the engine and maintain it, the sternum and costal cartilage had to be removed. In their place, I believe magical components/ magical versions of those body parts that allow for her to phase through them to get to her engine were added.
