Electric foils

The enhanced fencing foils with which Agatha and  Gil fend off the attack by Slaver wasps aboard Castle Wulfenbach -- sort of. They're important for three reasons. First, of course, they do keep Agatha and Gil alive, which counts for something. Second, they show Gil a little more about Agatha's qualifications as a Spark, and also give Agatha a chance to show a bit of Heterodyning, the humming that's Agatha's way of getting into the Sparky hyper-focus mode. Third, and important from a narrative perspective, by drawing power from the Heterodyne Device, they provide a hint that the Device is going to be something worth keeping an eye on in the future.