What is a knurlheim from the inheritance cycle?

Oh, dude, a knurlheim in the Inheritance Cycle is basically just a fancy way of saying "magic sWord." It's like if Excalibur and a lightsaber had a baby. So, yeah, it's a pretty cool weapon that can do some serious damage in the hands of the right person. Just don't try to bring it through airport security, they tend to frown upon magical weaponry.