Russia’s invasion of Ukraine has long conjured images of war that Europe thought had been consigned to the history books. And now Kyiv appears to have an eye-catching new weapon that adds a medieval twist to its defensive arsenal.

Ukraine’s forces have shared videos in the past week showing the use of drones that spew a fiery substance onto troops and terrain below, a possible boost in their bid to press forward with an assault on Russian territory while defending against Moscow’s…