Vampire Dragons are members of the undead. They sustain themselves by drinking the blood of others. It is said that they are only “alive” at night, and seem dead or asleep during the day, as they cannot endure sunlight for long periods of time. Vampire Dragons can only reproduce by changing the eggs of other dragons, puncturing the shell with their fangs and injecting venom that kills the baby inside.
Avians had the misfortune of growing up during one of the rare periods of time when a multitude of new dragons appeared. So he never really got paid any attention, since everyone was busy fawning over the Nocturnes and Canopies. Despite his neglect, he didn't even turn into a neglected dragon, possibly because he is too dead for such things. Shame, because being a neglected would have actually been interesting. Nobody really cared when he was a hatchling, and now he's just another unimpressive vamp in a veritable army. How utterly boring.