Basilisks are ancient beasts of old, and were belived to be extinct.
These monstrous serpents were the only known thing in the world who actually preyed upon Vampyres, and as such, were the only things in the world that Vampyres actually feared. This fear was so great, that thousands of years ago, the Vampyres formed large raids to hunt down these monsters, and over the years, effectively hunted them to extinction, thus eliminating the largest threat to their kind.
Basilisk venom is 100% fatal to Vampyres with even a minimal dose, and is universally fatal to everything else. The only known cure is to consume the heart of a Phoenix. Erykas managed to create an antidote.