Help please

[Dornish First Sword]

I recall reading a book many years ago that I didn't finish. I remember it had a white cover (was the first in a series aswell) and the "evil" creatures in it were flying gargoyle type creatures armed with swords that would extend an extra length from the hilt when a button was pressed. The main character was a prince or noble of some sort whos family was lost to these creatures.

Hope this information rings a bell with someone here.