Oh, is Byakuren talking about Myouren? Did she meet him here?
The Shigisan Engi, the picture scroll the historical Myouren's story is mostly known through, features a story where the monk Myouren's elder sister walks across the country to find him after not seeing him for 20 years (guided by a purple cloud). I.e. the very story Byakuren is (very loosely) based on is about their reunion, but one that took place before he died and she became a youkai.
Cute little retelling. He still slayed her, but she continues to mess with him regardless. Also kind of fun that he was so short he managed to hid in her "blindspot".