This isn't really an important theory, but it is something Chel and I have been pondering: What kind of dragon is the symbol thing? First of all, we all know that the compass rose on the map is, indeed, the Nightwings' island:

Pretty obvious; but what dragon is the symbol? Chel and I decided on either a Skywing or a Nightwing, but I am starting to think otherwise. Here is the symbol thing: 

I believe that the dragon in the symbol is Darkstalker. Here is a description of him from the book: "... he was long and frighteningly thin, with a narrow handsome face, twisted horns, and eyes as black as underground caverns." (pages 285-86, book 8) What do you think? 

-DuffelSnuff the Baka