Tamamo no Mae
I always wanted to do a series based (even if loosely) on Japanese folklore/traditions, this is the second in the series.
Tamamo-no-Mae was said to be an extremely beautiful and intelligent courtesan who could answer any question she was asked and was admired across Japan, even by the Emperor himself. When they eventually started a romance, the Emperor suddenly fell ill. It was later discovered that it's because Tamamo-no-Mae, in fact, was an evil kitsune (nine-tailed fox) spirit whose mission was to overthrow the Emperor.
Obviously later she met her terrible end for what she had done by the hands of one of the Emperor's best warriors, but that's a different story.