Vanitas of the Blue Moon is a vampire. According to legend, they were born under the Blue Moon and were the ostensible creator of the Book of Vanitas and the disease Malnomen. It was their intent to destroy vampires through the corruption of their True Name, hoping to take revenge for a lifetime of prosecution for their birth. This power would be passed onto their kin, prophesied to bring about the end of all vampires.
(Source: Mochizuki Jun Wiki)
