Dragons of the world of Ashan.

Nazgûl are not dragons. And they fly on wyverns (or at least, something similar). As for how Beowulf defeated the dragon, you should ask Beowulf himself. By the way, where is he?

You can read about how Beowulf defeated the dragon in the book. But you won't be able to ask him, because, according to the book, he died long before we were born, unfortunately.