It's a Rust Monster.
Imps are a kind of malevolent free spirit that has the power to possess magical foci. Like many of the things from Threats 2, it is astoundingly non-frightening. Essentially you have to put the focus aside until you can get enough friends together to kick it out of your focus and blow it to hell.
So it's a way for the gamemaster to screw with your equipment. Apparently we are supposed to consider them a world danger or something, but I just don't see it. It's like putting termites as major world dangers - they can potentially cost you hundreds of thousands of
in home repairs, but they just aren't a threat to wold peace.
-Frank