Nyx

Nyx is a goddess and the personification of night in Greek mythology. She is the daughter of Chaos and the mother of the gods Hypnos, Thanatos, and Erebus (the personifications of sleep, death, and darkness, respectively). She is reputedly feared by Zeus, the king of the gods. The one syllable, the x-ending (think Bex), and the potential to be a nickname for Phoenix make this somewhat mysterious, dark name, which sounds like "nothing," all the more alluring. (this is for Halloween)