Screw-worms
The larva of a special variety of flies, which were bred in the course of experiments by one crazy alchemist. It turned out that you can cross a fly and a screw if you are crazy enough and well versed in alchemy. They look like miniature screws, even have shade of metal or rust, with a head resembling the head of a screw. They usually curl into dead wood, especially rotten wood, and suck out nutrients from it, making it lighter and more brittle. Similar to dry rot. Although in the process, all other life that reproduces in the wood also dies and becomes food for the larvae: fungi, bacteria, some small enough arthropods... They are unusually hard for larva and has some pretty sharp edges - they will try to spin to cut anyone, who disturbs them. At least wood infested by them is harder to burn, or is it worse?