Vanitas is a legendary vampire born under the Blue Moon—a rare and significant event in vampire lore. Shunned by his own kind, he channeled his bitterness into creating the Book of Vanitas, a grimoire that essentially handed anyone who possessed it the power to wipe out all vampires. It was basically an act of spite taken to mythical extremes.
At some point, the original Vanitas found someone worthy (or desperate enough) to pass on his name and the Book to. This successor, also called Vanitas, became the new keeper of this world-ending artifact—a burden wrapped up in a legacy of revenge.
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