"That’s basically it. They’re too magically weak to survive without it. To kill an imp, all you need to do is cast it out to fend for itself – earth isn’t like our home, and they won’t survive here. They’ll slowly wither and go mad, and eventually they’ll die. That’s why the cruellest thing a wizard can do to an imp is to "set it free’ from its bond to them. Technically they’re then free from their old ancestral oath to serve, and could return to our home world, but they don’t have the power or knowledge to do so."